Water heater size calculator

What size water heater do I actually need?

Water heater sizing is one of the most miscalculated numbers in residential plumbing. The default contractor rule is "match what was there" or "1 gallon per person per bathroom plus 20", both of which routinely produce undersized units that run cold showers and oversized units that waste standby energy. The right answer comes from matching the heater's peak delivery to your household's peak hour demand.

For tank water heaters, the relevant capacity number is the First Hour Rating (FHR), not the gallons on the side of the tank. FHR is the gallons of hot water the unit can deliver in the first hour starting from a fully heated tank. A 40-gallon gas tank has an FHR around 70 gallons because the burner heats incoming water while you draw the stored hot water. A 40-gallon electric tank has an FHR of about 45 gallons because resistance elements recover slower than gas burners. The ENERGY STAR label on every water heater publishes the FHR. That is the number to shop on.

For tankless water heaters, the relevant capacity is BTU input rating, paired with the gallons-per-minute (GPM) the unit can sustain at a given temperature rise. There is no storage, so the unit has to heat all incoming water on demand. The math is direct: required BTU/hr equals peak GPM times temperature rise times 500. A household needing 6 GPM at 70 degree rise (50 F inlet, 120 F outlet) needs 6 times 70 times 500 equals 210,000 BTU/hr. That sizes a high-end residential gas tankless.

Peak hour demand: the number every water heater calculator should start with

DOE publishes a worksheet (10 CFR Part 430 Appendix E) for estimating peak hour demand by summing typical hot water uses during the busiest hour of the day. The standard defaults:

• Shower: 10 gallons of hot water per use
• Bath: 20 gallons per use
• Shave: 2 gallons per use
• Hand or face wash: 4 gallons per use
• Dishwasher: 14 gallons per cycle
• Food prep: 5 gallons
• Clothes washer (warm): 14 gallons per cycle

Run the worksheet for your busiest hour. A 4-person household with two morning showers, two hand washes, two shaves, and one dishwasher cycle running at 7 AM hits 20 + 8 + 4 + 14 equals 46 gallons of peak demand. Match a tank FHR of at least 46. For most 4-person households that lands at a 40-gallon gas tank or a 50-gallon electric tank. The "1 gallon per person" rule would have undersized this household by half.

Tankless GPM and the inlet temperature problem

Tankless sizing depends on incoming groundwater temperature, which varies by location. DOE published a US groundwater map showing inlet temperatures range from about 37 F in northernmost states to 75 F in south Florida and Hawaii. The calculator above looks up your design inlet by climate zone, but the underlying math is what matters:

• Zone 1 (Honolulu, Key West): ~75 F inlet, 45 F rise to a 120 F outlet
• Zone 2 (Orlando, Phoenix metro): ~70 F inlet, 50 F rise
• Zone 3 (Memphis, Charlotte): ~65 F inlet, 55 F rise
• Zone 4 (Albuquerque, Richmond): ~55 F inlet, 65 F rise
• Zone 5 (Portland OR, Cleveland): ~50 F inlet, 70 F rise
• Zone 6 (Madison, Spokane): ~45 F inlet, 75 F rise
• Zone 7 (Burlington VT, interior Alaska): ~40 F inlet, 80 F rise

A tankless that delivers 8 GPM in Florida only delivers about 5 GPM in Minnesota, because the unit cannot heat the colder incoming water as fast. Manufacturer product specifications usually list flow rates at multiple temperature rises, but the marketing emphasizes the warm-climate number. Read the cold-climate spec, not the warm-climate spec, before committing.

Electric tankless: the math that catches most homeowners

Electric tankless looks attractive in theory: no gas line, no venting, instant hot water. The math kills the idea in cold climates and for high-flow households. Resistance heating is limited by the panel and the wire. A typical large residential electric tankless is 27 kW at 240 V, which draws 113 amps split across multiple 40 amp circuits. At 70 F temperature rise (a Minnesota winter), 27 kW sustains only about 2.6 GPM, barely enough for one shower at full flow. Run a shower and a kitchen sink together and the outlet temperature drops below comfortable.

Whole-house electric tankless almost never makes sense in zones 5 and colder. The panel upgrade alone often costs $2,500 to $5,000 (200 to 300 amp service is typical). In zones 1 to 3 with mild inlet temperatures, a 27 kW unit can sustain 3.5 to 4.5 GPM and works for a 2 to 3 person household. Past that, gas tankless or a heat pump water heater becomes the better answer. The calculator above flags this automatically when the math fails for your zone and household.

Heat pump water heaters: the alternative that usually wins on lifetime cost

Heat pump water heaters (HPWH) move heat from surrounding air into the tank instead of generating it from fuel or resistance. They use 60 to 70 percent less energy than an electric resistance tank and 30 to 40 percent less than a gas tank in mild climates. The 30 percent / $2,000 federal tax credit (Inflation Reduction Act Section 25C) covers HPWH purchase plus installation, which usually nets a 3 to 6 year payback against an electric resistance tank.

HPWHs have one practical constraint: they need 700 to 1,000 cubic feet of unconditioned space around the unit so the air heat exchanger has thermal mass to pull from. A small mechanical closet usually fails this requirement. Garages (zones 1 to 5), basements, and large utility rooms work fine. In cold climates (zones 6 to 8), HPWHs still work but with reduced winter performance because the surrounding air is colder; most cold-climate installs use a hybrid mode that switches to resistance backup below 35 F ambient.

Recovery rate vs FHR and why both matter for tank sizing

FHR is the headline number, but recovery rate matters for the second hour and beyond. Recovery is the gallons per hour the heater can produce at steady state once the storage tank is depleted. Gas burners typically recover at 35 to 50 GPH at a 90 F rise; electric resistance at 20 to 25 GPH; HPWH at 8 to 10 GPH. A household that draws hot water in waves (back to back showers, then a dishwasher cycle 30 minutes later) needs recovery as much as FHR.

For tankless this concern does not exist. Tankless units run at full BTU indefinitely as long as the gas or electricity supply holds out. The trade-off is that tankless cannot serve simultaneous fixtures past its sustained GPM ceiling. Households that want both endless hot water AND multiple simultaneous fixtures need a tankless sized 30 to 50 percent above the simple peak GPM calculation, or a pair of tankless units in parallel.

Gas line, vent, and electrical service: the install gotchas

Sizing the unit is half the problem. The other half is what your house can support:

• Gas tankless: typically needs a dedicated 3/4 inch gas line for units up to 150,000 BTU/hr, 1 inch for larger. The existing 1/2 inch line to a 40,000 BTU tank will not carry a 199,000 BTU tankless. Plus stainless steel concentric vent (typically 4 inch) through an exterior wall.
• Electric tankless: needs 100 to 150 amps of dedicated panel capacity split across two or three 40 amp double-pole breakers. Often requires a 200 to 300 amp service upgrade.
• Heat pump water heater: needs a standard 30 amp 240 V circuit (same as electric resistance), plus the 700 to 1,000 cubic feet of surrounding space.
• Gas tank: the simplest swap. Existing gas line, vent, and water connections almost always work for a same-sized replacement.

Always factor install scope into the comparison. A 50-gallon gas tank that drops in for $2,500 versus a gas tankless that needs a new gas line and vent for $5,500 versus an HPWH that needs nothing but a circuit for $3,200 is a three-way comparison the calculator can not make for you. Ask three local contractors for itemized bids that include all the work.