Water Heater Sizing Calculator
The first-hour rating a tank needs for your busiest hour, or the power a tankless heater needs.
Result
140,203 BTU/h
Arithmetic from your figures; heaters lose some heat, so allow a margin and check the unit’s own rating.
- Temperature rise
- 70 °F (38.9 °C)
- Note
- Sizing arithmetic only: relief valves, venting, gas and electrical supply are for a qualified installer and local codes. Hotter water raises the risk of scalding.
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Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Water Heater Sizing Calculator
- Choose tankless or storage, and whether you measure in gallons or litres.
- For tankless, enter the hot water flowing at the same time, in gallons or litres a minute. For a tank, list what uses hot water in your busiest hour.
- Enter the incoming water temperature, which is lower in winter and in colder places, and the hot water temperature you want. Read the power or first-hour rating needed.
The physics
Heating water takes a fixed amount of energy per pound per degree: one BTU per pound per degree Fahrenheit, by the definition of the BTU. A US gallon of water weighs about 8.345 lb, so:
BTU per hour = gallons per minute × 60 × 8.345 × temperature rise (°F)kW = litres per minute ÷ 60 × 4.1868 × temperature rise (°C)That is exact, which is why the temperature rise matters so much: doubling the rise doubles the power, however good the heater.
Tank or tankless
A tankless heater has to supply the full power at the moment water flows, so it is sized by the flow of the fixtures running at once and the temperature rise. A tank heater stores hot water ahead of time, so it is sized by how much you use in your busiest hour, its first-hour rating. This page covers the arithmetic only; gas supply, venting, electrical capacity and relief valves are for a qualified installer and your local codes.
With your numbers
- Power = flow × 8.345 lb/gal × 1 BTU/(lb·°F) × temperature rise
Quick reference
Arithmetic from your figures; heaters lose some heat, so allow a margin and check the unit’s own rating.
| Job | Result |
|---|---|
| Tankless, 4 gpm, 50 to 120 °F | Heating power needed: 140,203 BTU/h |
| Tankless, 4 gpm, 70 to 120 °F | Heating power needed: 100,145 BTU/h |
| Tankless, 2 gpm, 50 to 120 °F | Heating power needed: 70,101 BTU/h |
| Tankless, 15 L/min, 10 to 49 °C | Heating power needed: 41 kW |
| Tank, two showers and a dishwasher | First-hour rating needed: 56 gal |
| 199,000 BTU/h unit at a 70 °F rise | Heating power needed: 140,203 BTU/h |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
2 sources
NIST Chemistry WebBook, water at 4 °C and 0.101325 MPa: 999.9749 kg/m³ (IAPWS-95), https://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185, rounded to the conventional 1000 kg/m³ of NIST SP 811 (2008) Appendix B, centimeter of water, conventional = 98.0665 Pa.
Used for
- The conventional density of water, 1000 kg/m³. Pure water at 4 °C, its densest, is 999.97 kg/m³.
NIST SP 811 (2008), Appendix B.9, p. 61: Btu_IT/(lb·°F) = 4.1868 E+03 J/(kg·K), in bold, so exact. https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication811e2008.pdf
Used for
- Specific heat of water: exactly 1 BTU per pound per degree Fahrenheit under the International Table BTU.
Common questions about the Water Heater Sizing Calculator
- Why does a tankless heater give less hot water in winter?
- Because it has to heat the water further. At 4 gallons a minute, heating from 70 °F to 120 °F takes about 100,000 BTU an hour; from 50 °F it takes about 140,000, 40% more. A heater that keeps up in a warm climate, or in summer, can fall short when the incoming water is cold. Size for the coldest water you expect.
- How do I size a tank water heater?
- By peak-hour demand, not household size. Add up the hot water used in your busiest hour: two 10-minute showers at 2.5 gallons a minute and a dishwasher using 6 gallons is 56 gallons. A tank’s first-hour rating, printed on its label, should be at least that. Two people who both shower at 7 am need more than four who stagger.
- What is a first-hour rating?
- How much hot water a storage heater can deliver in an hour starting with a full tank of hot water. It combines the tank size with how quickly the heater recovers, which is why it is a better guide than the tank’s gallons alone. It appears on the heater’s label.
- Does this page tell me what temperature to set?
- No. The hotter the water, the greater the scalding risk, and the right setting depends on your household and local rules. The page only works out what a given temperature takes.