Water Heater Size Calculator

How the water heater size estimate works

Tank: pick the hour of the day when the household uses the most hot water, count each use in that hour and multiply by the Department of Energy's average gallons per use: 20 for a shower, 2 for shaving, 3 for hand dishwashing or food prep, 7 for a dishwasher, 25 for a top-loading washer and 15 for a front-loader. The total is the peak hour demand. Look for a water heater whose first hour rating is at least that.

Tankless: a tankless unit has no stored hot water, so it is sized by flow. Add up the flow rates of the fixtures that run at once and work out the temperature rise, the output temperature minus the incoming water. The heat needed is GPM × 500 × rise, in BTU per hour; divide by the efficiency for a gas unit's input, and by 3,412 as well for an electric unit's kW.

Worked example, tankless: two showers at 2.5 GPM and a kitchen faucet at 2.2 GPM, 7.2 GPM. Incoming water at 50 °F and 120 °F out is a 70 °F rise. 7.2 × 500 × 70 = 252,000 BTU per hour of heat. At 81% efficiency, 252,000 ÷ 0.81 = 311,111 BTU per hour of gas input; electric, 252,000 ÷ 3,412 ÷ 0.91 = 81.2 kW. That is more than most single units deliver, which is why the fixtures in use at once matter so much.

Gallons of hot water per use

Average gallons of hot water per use, from the DOE's worksheet for estimating peak hour demand. The DOE notes that the figures are averages from a variety of published sources; a long shower or a large tub uses more, which is what the "other" field is for.

Fixture flow rates for tankless sizing

The calculator fills in the federal maximum flow for each fixture made since 1994; WaterSense-labeled fixtures use less. A fixture mixes hot and cold, so the hot share is lower than the full flow: the DOE's own example counts a faucet at 0.75 GPM. Typing the full flow is the safe side. Measure a fixture by timing how long it takes to fill a bucket.

Temperature rise and the incoming water

A tankless water heater is rated by the temperature rise it can give at a given flow, so the incoming water temperature changes the size. The DOE suggests assuming 50 °F unless you know otherwise, and 120 °F out for most uses. The colder the water coming in, the bigger the rise and the fewer gallons a minute a unit can heat.

The calculator has no table of water temperatures by region: none of the sources gives one. Measure it: run the cold tap and hold a thermometer under it, as the DOE suggests, and use the winter figure if you can, since that is when the rise is biggest.

Gas BTU, electric kW and efficiency

One BTU heats a pound of water by 1 °F, and a gallon of water weighs about 8.34 pounds, so one gallon a minute for an hour is about 500 pounds: GPM × 500 × rise is the heat in BTU per hour. One kilowatt-hour is 3,412 BTU.

The efficiency fields start at the federal minimum uniform energy factor for tankless water heaters: 0.81 for gas units over 50,000 BTU per hour and 0.91 for electric ones. ENERGY STAR gas tankless units are 0.95 or more. The uniform energy factor is a seasonal figure, so using it gives an input on the safe side; type the efficiency from the unit's spec sheet if you have it, and check the unit's own flow chart at your rise.

What the plumber checks

This page is a rule of thumb, not a sizing service. A licensed plumber confirms the size and what the local code asks for: the gas line and venting for a gas unit, the electrical service and breakers for an electric one, the relief valve and the expansion tank. A tankless unit's real output at your rise is in its spec sheet; use the calculator to get a first number and to know what to ask.

For plumbing and HVAC reps: what to write down on the consult

The size comes from the hour when everyone showers. Ask who is in the house, when they shower, whether two showers run at once, whether the dishwasher or washer runs in that hour, how old the heater is and whether it has run out of hot water. Write it down while you are in the house, and read it back before you send the quote.

Sources, each read on 2026-10-27. All results are rule-of-thumb estimates:

• U.S. Department of Energy, Energy Saver: Sizing a New Water Heater (worksheet, first hour rating, tankless method; archived copy of 2 July 2026, since removed from energy.gov)
• 10 CFR 430.32: federal maximum showerhead and faucet flow, and minimum water heater efficiency (Cornell LII)
• EPA WaterSense: Showerheads (2.0 GPM labeled)
• EPA WaterSense: Bathroom Sink Faucets (1.5 GPM labeled)
• U.S. EIA: British thermal units (one pound of water by 1 °F; 1 kWh = 3,412 BTU)
• USGS Water Science School: Water density (62.4 pounds per cubic foot)
• ENERGY STAR: Residential water heaters key product criteria (gas tankless UEF 0.95)

On the same visit? The furnace size calculator and the BTU and AC size calculator size the heating and cooling, and the water softener size calculator sizes the softener that protects the new heater from scale.

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