Water Heater Size Calculator
Tank size from peak-hour demand, or tankless kW and BTU.
About this tool
For a tank, enter how many showers, baths, sink uses and appliance loads happen in your busiest hour. For tankless, enter what might run at the same time and your incoming and target water temperatures. The tool gives the peak-hour demand, or the flow, electric power and gas input needed.
Sizing tank and tankless heaters
A storage tank heater should match your peak-hour demand, the most hot water used in one hour, which is what the first-hour rating (FHR) on US EnergyGuide labels measures. Typical figures: about 38 L (10 gal) per shower, 76 L (20 gal) per bath, 23 L (6 gal) per dishwasher load. Two showers, two sink uses and a dishwasher in the same hour come to about 115 L (30 US gal), so look for a heater with an FHR of at least that, usually a 30โ40 gallon tank. A tankless (on-demand) heater is sized by flow and temperature rise instead. Add the flow of everything that could run at once (a shower about 7.6 L/min or 2 gpm, a tap about 4 L/min) and multiply by the rise from your coldest incoming water to the roughly 49 ยฐC (120 ยฐF) you want. One shower plus a tap at a 39 ยฐC rise needs about 31.6 kW (around 108,000 BTU/h of output). That's why whole-house electric tankless units need very large electrical supplies, and why gas models are common in cold climates. Honest limits: usage per fixture varies with low-flow fittings and habits. Check your model's FHR or flow chart, and local codes for gas venting and electrical capacity. Not plumbing advice. The watts to amps calculator and the specific heat calculator are related.
Frequently asked questions
What size water heater do I need?
One whose first-hour rating covers your busiest hour, roughly 30 to 40 gallons for 2 to 3 people and 50 or more for larger households.
What is a first-hour rating?
How many gallons of hot water a tank heater can supply in an hour starting full; it's on US EnergyGuide labels.
How do I size a tankless water heater?
Add up the flow of fixtures used at once (gpm or L/min) and the temperature rise needed; kW โ L/min ร rise ยฐC ร 0.07.
Why does a tankless heater need so much power?
Heating water instantly takes a lot of energy: one shower at a 35 ยฐC rise needs about 18 kW.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.