What Size Water Heater Do I Need? Tank and Tankless Sizing Guide

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.