Water Heater Size Calculator
Size a tank or tankless water heater. Tanks by the first-hour rating your busiest hour needs; tankless by flow and temperature rise, in gas BTU/h or electric kW and amps.
How to Use
- Choose a tank or a tankless heater.
- For a tank: count how many of each use happen in your busiest hour, usually the morning.
- For tankless: enter how many of each fixture can run at the same time, and the temperature rise from the incoming water to the hot setting.
- For tankless gas, choose condensing (about 95% efficient) or non-condensing (about 82%).
- Read the first-hour rating or the size of tank, or the gas input, electric kW and amps.
Worked Example
Tank: a family of four on a weekday morning. Three showers at 10 gal, one shave at 2, washing hands and face at 4, making breakfast at 5 and running the dishwasher at 6: 30 + 2 + 4 + 5 + 6 = 47 gal in the peak hour. The heater’s first-hour rating must be at least 47 gal. A 30 gal gas tank with a 32,000 BTU/h burner gives about 0.7 × 30 + 32,000 × 0.76 ÷ (8.34 × 90) = 21 + 32 = 53 gal; electric needs a 40 gal tank, about 48 gal.
Tankless: two showers at once, 50 °F water heated to 120 °F. Flow: 2 × 2.0 = 4 gpm. Heat: 500.4 × 4 × 70 = 140,100 BTU/h of output. A condensing gas unit at 95% needs 147,500 BTU/h input, so a 160,000 BTU/h model. Electric: 140,100 ÷ 3,412 ÷ 0.99 = 41.5 kW, 173 A at 240 V, more than most homes’ whole service.
The common mistake: sizing a tank by family size. “50 gallons for four people” ignores when the water is used. A household that showers in the evening and runs the washer in the morning needs far less than one where everyone showers between 7 and 8.
Show Work
Formulas
From Kettles to Condensing Tankless
Hot water on tap is younger than electric light in many homes. Edwin Ruud, a Norwegian-born engineer in Pittsburgh, built one of the first automatic storage water heaters around 1889, and the insulated tank with a thermostat became standard in American houses in the 20th century. In Europe and Japan, instantaneous gas heaters, the ancestors of today’s tankless units, were common from the early 1900s.
The US Department of Energy now rates tanks by first-hour rating and uniform energy factor, printed on the yellow EnergyGuide label, and tankless units by maximum gpm at a given rise. Heat-pump water heaters, which move heat from the surrounding air, use roughly a third of the electricity of a standard electric tank but recover more slowly.
About This Calculator
This calculator sizes a tank water heater from your busiest hour using the US Department of Energy method, and a tankless heater from the fixtures running at once and the temperature rise, giving gas input in BTU/h and electric power in kW and amps, in US or metric units. It charts demand against capacity.
Everything runs in your browser; nothing is sent anywhere. Tank first-hour ratings here are conservative estimates for standard models; the EnergyGuide label gives the exact figure. Electric tankless units need a large dedicated supply; have an electrician check your panel.
Related calculators: BTU Calculator, Electricity Cost Calculator, and Pool Volume Calculator.
Frequently Asked Questions
What size water heater do I need?
For a tank, one whose first-hour rating (FHR) covers your busiest hour. Add up the hot water used then: 10 gal per shower, 20 per bath, 6 for a dishwasher, 7 for a clothes washer and so on. Three showers, a shave, washing hands, breakfast and the dishwasher make 47 gal, so look for an FHR of at least 47 on the yellow EnergyGuide label.
What is the first-hour rating?
The number of gallons of hot water the heater can supply in an hour starting full. It combines the usable stored water with what the burner or elements reheat in that hour, so a 40 gal gas tank, with its fast burner, typically rates about 70 gal, while a 50 gal electric tank rates about 60. It is a better guide than tank size alone.
How do I size a tankless water heater?
By flow and temperature rise. Add up the flow of fixtures used at once, about 2 gpm per shower, 1.5 per kitchen faucet, 1 per bathroom faucet. Then the heat needed is 500 × gpm × rise in °F. Two showers at 4 gpm with a 70 °F rise need 140,000 BTU/h of output: a 160,000 BTU/h condensing gas unit, or 41 kW electric, which is more than most homes can supply.
What temperature rise should I use?
The hot setting, usually 120 °F (49 °C), minus the incoming water temperature. Groundwater ranges from about 40 °F in the northern US to 75 °F in the south, so the rise is 45 to 80 °F; colder water needs a bigger tankless heater. Winter inlet temperatures set the size.
Gas or electric tankless?
A gas tankless heater can deliver 5 or more gpm at a 70 °F rise from one 199,000 BTU/h unit. Electric units need very large circuits: 27 kW draws 113 A at 240 V and still only manages about 2.6 gpm at that rise. Whole-house electric tankless suits warm climates, small homes or point-of-use.
How do I use the Water Heater Size Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Family of four
3 showers, shaving, hands, breakfast, dishwasher: 47 gal peak hour, a 30 gal gas or 40 gal electric tank.
Busy household
4 showers, two hand-washes, breakfast and the dishwasher: 59 gal, a 40 gal gas or 65 gal electric tank.
Two showers tankless
4 gpm at a 70 °F rise: 147,000 BTU/h condensing gas, or 41 kW electric.
Warm climate
The same two showers with a 45 °F rise: 95,000 BTU/h gas or 27 kW electric.
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