Category: Buying Guides

Last updated: October 2026

Most households need a 30 to 40-gallon tank for 1 to 2 people, a 40 to 50-gallon tank for 3 to 4 people, and a 50 to 80-gallon tank (or a tankless unit) for 5 or more, but the number that really matters is the first-hour rating (FHR), which should match your busiest hour of hot water use. For a tankless water heater, size by gallons per minute (GPM): add up the fixtures you run at the same time (typically 4 to 6 GPM for a family of four running two showers) and make sure the unit delivers that flow at your climate’s temperature rise. A family of 4 typically needs a 50-gallon tank with an FHR of at least 65 to 80 gallons, or a gas tankless unit of around 180,000 to 199,000 BTU.

Quick water heater size chart

These are starting points based on manufacturer guidance (Rheem recommends 30 to 40 gallons for 1 to 2 people, 40 to 50 for 2 to 4, 50 to 55 for 4 to 5, and 75 gallons or tankless for 5+). Electric tanks recover more slowly than gas, so they’re usually sized one step larger. Confirm your size with the peak-hour method below.

How to size a tank water heater: first-hour rating

Tank size in gallons is only part of the story. Two 50-gallon heaters can deliver very different amounts of hot water in a busy hour, because one may have a bigger burner or stronger elements that reheat water faster. That’s why the US Department of Energy recommends sizing storage water heaters by first-hour rating: the number of gallons of hot water the heater can supply in one hour, starting with a full tank.

You’ll find FHR on the yellow EnergyGuide label (listed as “Capacity (first hour rating)”) and on the manufacturer spec sheet. Heat pump water heaters often don’t show FHR on the label, so check the product literature.

Step 1: Find your peak hour

Think about the single busiest hour in your home. For most families it’s weekday mornings (showers back to back) or evenings (showers, dishwasher, laundry).

Step 2: Add up gallons used in that hour

Use DOE’s average hot water use per activity:

Low-flow showerheads and shorter showers reduce the shower figure. A deep soaking tub can need far more: small tubs hold about 40 gallons and large soaking tubs up to 140 gallons, according to Ruud.

Step 3: Choose a heater with an FHR at or above that total

DOE’s own example: three showers (60 gallons), one shave (2 gallons) and one hand dishwashing (3 gallons) add up to 65 gallons in the peak hour, so that household needs a water heater with an FHR of at least 65 gallons. Pick an FHR within a few gallons of your peak demand; oversizing wastes energy through standby loss.

Typical first-hour ratings by tank size

FHR ranges are typical of current residential models from major brands; always check the exact model. Recovery figures are calculated: a 40,000 BTU burner at about 78% efficiency heats roughly 41 gallons per hour by 90°F, while a 4,500-watt element heats about 20 gallons per hour by the same amount. That slower recovery is why an electric 50-gallon tank delivers less in the first hour than a gas 50-gallon tank.

Water heater size for a family of 4: worked examples

Example 1: Typical morning rush

Right size: a 40 or 50-gallon gas tank with an FHR of about 65 to 80 gallons, or a 65-gallon electric or heat pump tank. A standard 50-gallon electric tank (FHR around 60 to 65) is borderline.

Example 2: Evening with laundry and dishwasher

Right size: a high-input 50-gallon gas tank (FHR 100+), a 75-gallon gas tank, or a whole-house tankless unit. The simplest fix is often behavioral: run laundry and the dishwasher outside shower time, which drops demand to 80 gallons.

How to size a tankless water heater: GPM and temperature rise

Tankless heaters don’t store water, so gallons and FHR don’t apply. You need two numbers:

1. Peak flow rate (GPM): the total gallons per minute of every hot water fixture you’d realistically run at the same time.
2. Temperature rise: your desired outlet temperature (usually 120°F) minus your coldest incoming water temperature.

Then choose a unit rated to deliver your peak GPM at that rise. DOE notes that a 70°F rise is often possible at about 5 GPM with gas tankless models and about 2 GPM with electric models.

Step 1: Add up fixture flow rates

If you don’t know a fixture’s flow rate, DOE suggests timing how long it takes to fill a container for one minute. Example: one 2.5 GPM shower plus a 0.75 GPM faucet means you need at least 3.25 GPM.

Step 2: Find your temperature rise

Incoming water temperature varies a lot across the US and drops in winter. Size for the coldest month, not the average. Ruud notes summer inlet temperatures are often 20°F to 25°F warmer than winter.

Based on Ruud’s winter rules of thumb (northern tier 40°F, most of the South 50°F, Southern California, the Southwest and Gulf states 60°F) and Rinnai’s groundwater map (75°F for Hawaii and Puerto Rico). DOE assumes 50°F if you don’t know. Well water and long buried supply lines can be colder.

Step 3: Convert to BTU (gas) or kW (electric)

Use these standard formulas (500 ≈ 8.33 lb per gallon × 60 minutes):

• Gas BTU input needed = GPM × 500 × temperature rise ÷ efficiency
• Electric kW needed = GPM × 500 × temperature rise ÷ 3,412

Most whole-house gas tankless units top out around 199,000 BTU, and the largest common residential electric units are about 27 to 36 kW. So in a cold climate, one gas unit handles about two showers, and an electric unit is realistically a one-shower-at-a-time solution. That matches Consumer Reports’ testing, where electric tankless units struggled to reach 120°F with 58°F incoming water but performed fine with 74°F water.

What size tankless water heater do I need? By household

Electric tankless: check your amps first

Electric tankless sizing is limited by your electrical service, not just water needs. Amps needed ≈ watts ÷ 240 volts:

For example, the Rheem RETEX-27 requires three 40-amp double-pole breakers. Most homes need 200-amp service for a whole-house electric tankless unit. Electric tankless makes the most sense in warm-groundwater states.

Gas vs electric vs heat pump: how fuel changes the size

• Gas tanks recover roughly twice as fast as standard electric tanks, so a 40-gallon gas tank can match or beat a 50-gallon electric tank in the first hour.
• Electric tanks should usually be one size larger than the gas equivalent for the same family.
• Heat pump water heaters recover slowly in efficiency mode. Many installers size one step up (for example, 65 gallons instead of 50 for a family of four) and use “hybrid” mode during peak periods. See our heat pump water heater cost guide.
• Tankless gas is sized by BTU and GPM; electric tankless by kW and available amps.

Compare fuel costs in our gas vs electric water heater replacement cost guide, or weigh the two technologies in tankless vs tank water heater.

What happens if your water heater is the wrong size?

Too small

• Last person in the shower gets lukewarm water
• Can’t run the dishwasher or washer during showers
• Owners turn up the thermostat to compensate, raising scald risk and energy use
• For tankless: temperature drops when a second fixture opens

Too big

• Higher purchase and installation cost
• More standby heat loss, which raises bills every month
• May not fit the existing space or need a new gas line or venting
• For tankless: higher minimum fire rate can cause temperature swings at low flow

If you’re replacing an old heater and never ran out of hot water, the same size is usually right unless your household has grown. If your old tank is leaking, see our water heater leaking guide; if it’s just old, check our water heater lifespan guide.

Water heater cost by size

Ranges from Angi’s water heater installation cost data and ConsumerAffairs’ tankless installation data. Upsizing can add hidden costs: a 75-gallon gas tank may need a larger vent, and a tank that’s taller or wider than the old one may not fit through the door or closet. For detailed numbers see our water heater replacement cost, 50-gallon water heater replacement cost and tankless water heater cost guides, plus installation labor cost.

Sizing tips plumbers use

• Size for winter. Groundwater is coldest in January to March; that’s your design case for tankless.
• Count real overlaps, not every fixture. Few homes run every tap at once. Two showers plus a faucet is a realistic peak for most families.
• Swap showerheads before upsizing. Changing from 2.5 GPM to 1.8 GPM heads cuts peak demand by about 28%.
• Consider a mixing valve. A thermostatic mixing valve lets you store water at 140°F and blend it down to 120°F at the taps, effectively increasing usable tank capacity and reducing Legionella risk. It must be installed correctly to prevent scalding.
• Check the soaking tub. A large tub can need more hot water than the whole rest of the household’s peak hour.
• Measure the space. Note height, diameter and door widths before buying a bigger tank.

FAQs

What size water heater do I need for a family of 4?

Most families of four need a 40 to 50-gallon gas tank or a 50 to 65-gallon electric tank with a first-hour rating of around 65 to 80 gallons. For tankless, plan on about 4.5 to 6 GPM, which usually means a gas unit of roughly 180,000 to 199,000 BTU.

Is a 40-gallon water heater enough for a family of 4?

A 40-gallon gas tank can be enough if its first-hour rating is around 65 gallons or more and showers are spread out. A 40-gallon electric tank usually isn’t, because it recovers much more slowly.

How many showers can you take with a 50-gallon water heater?

Using DOE’s average of about 20 gallons of hot water per shower, a 50-gallon gas tank with an FHR of 80 to 90 gallons can supply about four showers in an hour. A 50-gallon electric tank with an FHR of about 60 to 65 gallons covers roughly three.

What size tankless water heater do I need for a 3-bedroom house?

It depends on bathrooms and climate rather than bedrooms. A 3-bedroom, 2-bath home running two showers at once typically needs about 5 GPM. In most of the US that’s a gas tankless unit of about 180,000 to 199,000 BTU; in warm southern climates a smaller unit may work.

How many GPM do I need for a tankless water heater?

Add up the flow rates of fixtures you’ll run at the same time. One shower plus a faucet is about 3 to 3.5 GPM; two showers is about 4 to 5 GPM; two showers plus a kitchen faucet is about 5.5 to 6.5 GPM. Then confirm the unit delivers that GPM at your winter temperature rise.

Is a bigger water heater better?

No. An oversized tank costs more upfront and loses more heat while it sits idle. Size to your peak-hour demand using first-hour rating, not to the largest tank that fits.

What is a good first-hour rating?

A good FHR is one that matches your peak-hour hot water use. Roughly 40 to 55 gallons suits 1 to 2 people, 65 to 80 gallons suits a family of four, and 90+ gallons suits large households.

Can I replace a 50-gallon water heater with a 40-gallon?

Yes, if a 40-gallon model’s first-hour rating still covers your peak hour. Downsizing works best with a gas model, a high-input burner, or low-flow showerheads. If you often ran out of hot water before, don’t downsize.

What size water heater do I need for 2 people?

Two people typically need a 30 to 40-gallon gas tank or a 40 to 50-gallon electric tank. If you have a large soaking tub, size up. For tankless, about 2.5 to 3.5 GPM is usually enough.

Does incoming water temperature matter for tank water heaters?

Yes, but less than for tankless. Colder incoming water slows recovery, so a tank in Minnesota delivers somewhat less hot water per hour than the same tank in Florida. First-hour ratings are measured under standard test conditions, so cold-climate homes may want a little extra margin.

Last updated: October 2026

A tank water heater usually costs about $600 to $2,700 installed, while a tankless water heater typically runs $1,400 to $3,900 for a like-for-like swap and $4,000 to $10,000 or more when converting from a tank requires a new gas line, venting or electrical upgrades. Tankless units use roughly 24% to 34% less energy in homes that use 41 gallons of hot water a day or less, and they last longer (20+ years versus 10 to 15), but the energy savings alone rarely pay back the higher upfront cost quickly. Choose tankless if you plan to stay in the home a long time, have gas service with adequate capacity, or need endless hot water; choose a tank if you want the lowest upfront cost, have hard water, or need a simple replacement.

Tankless vs tank at a glance

How each type works

A tank water heater stores 30 to 80 gallons of water and keeps it hot around the clock using a gas burner or electric elements. When you open a tap, hot water leaves the top of the tank and cold water refills the bottom. Because the stored water slowly loses heat through the tank walls (called standby loss), the heater cycles on even when no one is using water.

A tankless water heater heats water only when a tap opens. A flow sensor detects water moving through the unit (usually once flow reaches about 0.4 to 0.5 gallons per minute), fires the burner or electric elements, and heats the water as it passes through a heat exchanger. There’s no stored water, so there’s no standby loss. The trade-off is that the unit can only heat so many gallons per minute, and that number drops as your incoming water gets colder.

For a deeper look at fuel choice, see our guide to gas vs electric water heater replacement cost.

Upfront cost: tankless vs tank

The equipment price gap is smaller than most people expect. A 50-gallon gas or electric tank costs roughly $500 to $1,200 at retail, and whole-house tankless units run about $525 to $1,500 or more for mainstream gas models. The big difference is labor and the work around the unit.

These ranges come from national cost guides (Angi, Bob Vila, Consumer Reports) and vary widely by region. For a full breakdown of tank pricing, read our water heater replacement cost guide or the 50-gallon water heater replacement cost page. For tankless-specific pricing, see tankless water heater cost.

Conversion cost: switching from a tank to tankless

Replacing an existing tankless unit with another tankless unit is straightforward. Converting from a tank is where budgets blow up, because a tankless heater demands much more fuel or power at once. A typical gas tank burner is rated around 40,000 BTU per hour; a whole-house gas tankless unit is commonly rated up to 199,000 BTU per hour. Electric tankless units commonly need 18 to 36 kW, which is far more than the single 30-amp circuit that feeds most electric tanks.

ConsumerAffairs puts a moderate same-fuel tank-to-tankless retrofit at about $3,900 to $6,000, an electric installation with electrical upgrades at $4,500 to $8,000, and a full fuel conversion at $6,000 to $10,500. Ask any installer to price the gas line, venting and electrical work as separate line items so you can see exactly where the money goes. Our water heater installation labor cost guide explains typical plumber rates.

Electric tankless: the panel problem

Electric tankless is cheap to buy but often expensive to wire. A popular 27 kW whole-house model (Rheem RETEX-27) requires three 40-amp double-pole breakers, which works out to roughly 112 amps at 240 volts. Many older homes with 100-amp or 150-amp service can’t add that load without a panel or service upgrade. Get an electrician to run a load calculation before you buy.

Operating cost and efficiency (UEF)

Water heater efficiency in the US is rated by Uniform Energy Factor (UEF), the federal test metric that replaced the old Energy Factor. Higher is better. You’ll find UEF on the spec sheet and EnergyGuide label, along with an estimated yearly energy cost.

The UEF ranges above are typical values from manufacturer spec sheets and ENERGY STAR criteria; check the exact model you’re quoted.

What the government and independent testing say

The US Department of Energy estimates that a tankless heater can be 24% to 34% more energy efficient than a conventional storage tank for homes using 41 gallons or less of hot water daily, and 8% to 14% more efficient for homes using around 86 gallons a day. In other words, the savings shrink as your household uses more hot water, because standby loss becomes a smaller share of total energy use.

Consumer Reports’ testing found similar real-world numbers. In its tests, gas tankless units cost about $195 a year to run versus about $245 for a gas tank, and electric tankless about $535 a year versus $580 for an electric tank. That’s a saving of roughly $45 to $50 a year at the energy prices CR used at the time. CR concluded that a gas tank-to-tankless swap could take around 22 to 27 years to pay back, while a heat pump water heater (about $240 a year to run in the same testing) paid back far faster for electric homes.

Payback examples (illustrative)

The examples below are illustrative only. They use simple assumptions, not quotes or measured bills: 64 gallons of hot water a day heated by 67°F (similar to the federal test conditions), natural gas at $1.50 per therm and electricity at $0.17 per kWh. Your energy prices, usage and installation costs will differ.

Two lessons stand out. First, a gas tankless upgrade can pay back within its lifespan if the install is simple, but conversion work can push payback past the life of the unit. Second, electric tankless saves very little over an electric tank because both already convert nearly all the electricity to heat. If you’re on electricity and want lower bills, a heat pump water heater usually beats tankless by a wide margin. (The federal 25C tax credit that covered heat pump and some gas water heaters ended for equipment installed after December 31, 2025, but many utilities and states still offer rebates.)

Tankless can still come out ahead on total cost of ownership because it lasts longer. If a tankless unit runs 20 years and you’d otherwise buy two tanks in that time, the second tank purchase (roughly $1,000 to $2,500 in today’s money) counts toward tankless payback.

Lifespan and long-term value

DOE notes that most tankless heaters have a life expectancy of more than 20 years and use easily replaceable parts, compared with 10 to 15 years for storage tanks. Tankless warranties vary by brand and often require professional installation and registration, and some reduce coverage if you don’t descale in hard water areas. Read the warranty terms before you buy. Our water heater lifespan guide covers warning signs that a tank is near the end.

Flow rate (GPM) and temperature rise by climate

This is the factor that decides whether a tankless unit will satisfy your household. A tankless heater’s capacity is measured in gallons per minute (GPM) at a given temperature rise: the difference between your incoming groundwater and the temperature you want at the tap (usually 120°F).

The colder your groundwater, the bigger the rise and the fewer gallons per minute the unit can deliver. DOE says a 70°F rise is often achievable at about 5 GPM for gas tankless units and only about 2 GPM for electric units.

The regional inlet temperatures are winter rules of thumb from Ruud and Rinnai sizing guidance. GPM figures are calculated with the standard formula (GPM = BTU input × efficiency ÷ (500 × temperature rise)), assuming 0.95 efficiency for the gas unit; electric figures use 27 kW × 3,412 BTU/kW. Rinnai’s own groundwater chart for its 199,000 BTU RX199 shows a similar spread, from about 4.6 GPM at 35°F groundwater to about 8.1 GPM at 72°F.

What that means in practice: a standard showerhead is limited to 2.5 GPM under federal rules, and WaterSense showerheads use 2.0 GPM or less. In a northern state, one 27 kW electric tankless unit can comfortably run about one shower at a time in winter. A 199,000 BTU gas unit can handle two showers at once in most of the country. To check your exact numbers, use our guide to what size water heater you need.

The “cold water sandwich” and other tankless quirks

Tankless units have a few behaviors that surprise people who’ve only owned tanks:

• Cold water sandwich. If you turn off the shower and turn it back on a minute later, you can get hot water (left in the pipes), then a short burst of cold while the unit re-fires, then hot again. Recirculation systems and small buffer tanks reduce this.
• Minimum flow rate. Most units won’t fire until flow reaches about 0.4 to 0.5 GPM. A trickle from a low-flow faucet may stay cold.
• Wait time. Tankless doesn’t deliver hot water to the tap any faster. You still wait for cold water in the pipes to clear, plus a few seconds for the unit to ignite. A recirculation pump (built into some models) solves this at the cost of some energy use.
• Simultaneous use. Running the dishwasher or washing machine during a shower can push an undersized unit past its GPM limit, dropping the shower temperature.
• Power outages. Gas tankless units need electricity for controls and the fan, so they stop working in a power outage. Many gas tanks with standing pilots keep heating without power.

Tanks have their own quirk: you can run out. A 40-gallon tank after three back-to-back showers may leave the fourth person with lukewarm water until it recovers.

Space and installation location

Tankless heaters mount on a wall and are roughly the size of a carry-on suitcase, freeing up the 2 to 3 square feet of floor a tank occupies. Many gas models can be installed outdoors in mild climates, which also frees up the utility closet and removes indoor venting. That makes tankless popular in condos, townhouses and small homes.

Tanks need floor space, clearance, a drain pan in many locations and, in seismic zones such as California, earthquake straps. Moving to a larger tank can be impossible in a tight closet, which is one reason homeowners with growing families switch to tankless.

Maintenance requirements

Hard water is the big variable. Scale (mostly calcium carbonate) insulates the tankless heat exchanger, cuts efficiency and triggers error codes. If your water is above about 7 grains per gallon, budget for annual descaling or a water softener or scale-reduction system. Tank owners should follow our step-by-step guide on how to flush a water heater.

Who should choose a tank water heater

• You want the lowest upfront cost. A like-for-like tank swap is almost always the cheapest option, often done in a few hours.
• Your heater just died. Emergency replacements are faster with a tank because no gas, vent or electrical upgrades are needed.
• You may sell within 5 to 7 years. You likely won’t recover tankless conversion costs through energy savings in that time.
• You have very hard water and don’t want annual descaling.
• You’re on electricity in a cold climate. An electric tank (or better, a heat pump tank) handles high winter demand without a massive electrical upgrade.
• Your household uses lots of hot water at predictable times. A properly sized tank with a good first-hour rating serves several showers in a row well.

Who should choose a tankless water heater

• You plan to stay 10+ years. The longer lifespan and lower energy use add up over time.
• You already have adequate gas service. If your gas line and meter can handle ~199,000 BTU, the conversion cost drops sharply.
• You’re short on space. Wall-mount or outdoor installation frees up a closet.
• You run out of hot water. Large families, soaking tubs or back-to-back showers benefit from continuous supply (if the unit is sized for your peak GPM).
• Your hot water use is low or irregular. DOE’s figures show the biggest percentage savings for homes using 41 gallons a day or less.
• You live in a warm climate. Warmer groundwater means more GPM from the same unit, and electric tankless becomes more practical.

Consider a heat pump water heater too

If you’re on electricity, the choice isn’t just tank vs tankless. A heat pump water heater typically uses a fraction of the electricity of a standard electric tank and usually pays back fastest. It needs space (often a room of around 700 cubic feet or a vented closet) and works best in a garage or basement that stays above roughly 40°F to 50°F. See our heat pump water heater cost guide.

Side-by-side decision checklist

If your existing tank is leaking right now, start with our water heater leaking guide to stop damage first, then compare options. If the heater still works but has a problem, check the water heater repair cost guide before replacing.

FAQs

Is a tankless water heater worth it?

It can be if you plan to stay in the home for 10 or more years, have gas service that can handle the unit, and the installation doesn’t need expensive gas line, venting or electrical upgrades. Energy savings typically run tens of dollars to around $100 a year, so the longer lifespan and continuous hot water are usually bigger reasons to buy than the bill savings.

What is the downside of a tankless water heater?

The main downsides are higher upfront and conversion costs, a limited flow rate that can struggle with several fixtures at once in cold climates, annual descaling in hard water, the “cold water sandwich” effect, and gas models not working during power outages.

How long do tankless water heaters last compared to tank?

DOE estimates most tankless units last more than 20 years, versus about 10 to 15 years for storage tanks. Regular descaling is the main factor in reaching that lifespan.

Do tankless water heaters really save money?

They save energy, but the dollar amount is modest. DOE estimates 24% to 34% energy savings for homes using 41 gallons a day or less and 8% to 14% for homes using around 86 gallons. Consumer Reports’ testing showed savings of roughly $45 to $50 a year versus comparable tanks at the prices it used.

How much does it cost to convert from a tank to a tankless water heater?

Expect roughly $3,900 to $6,000 for a typical same-fuel conversion and up to $10,500 or more if you need a new gas line, new venting, or an electrical service upgrade. A simple tankless-for-tankless replacement is far cheaper, typically $1,400 to $3,900.

Can one tankless water heater run a whole house?

A whole-house gas unit rated around 180,000 to 199,000 BTU can usually run two showers at once in most US climates. Homes with three or more bathrooms in use at the same time, or in very cold regions, may need two units linked together or a tank instead.

Do you run out of hot water with a tankless water heater?

No, you won’t run out the way you can with a tank, but if you exceed the unit’s GPM capacity the water temperature will drop. Proper sizing for your peak simultaneous use prevents this.

Is tankless better for a family of 4?

A family of four can be well served by either a 50-gallon tank or a whole-house gas tankless unit. Tankless makes sense if showers often overlap or happen back to back and you have adequate gas service; a 50-gallon tank with a first-hour rating around 65 to 80 gallons is cheaper and works well for most families of four.

Does a tankless water heater increase home value?

Buyers may see tankless as an upgrade, but there’s no reliable data showing it adds more value than it costs. Treat it as a comfort and efficiency upgrade rather than an investment you’ll recover at sale.

Do tankless water heaters need maintenance?

Yes. Most manufacturers recommend descaling the heat exchanger about once a year (more often in hard water) and cleaning the inlet filter. A professional descale typically costs $150 to $300.