Tankless Water Heater Size Calculator
Work out how big a tankless water heater needs to be. Enter the showers and faucets that run at the same time, the temperature of the incoming water and the outlet setting, and the calculator returns the hot water flow and the heater input in BTU/h and kW, plus the current an electric unit would draw.
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How It Works
A tankless heater has to warm the water as it flows, so its size is set by the peak flow and the temperature rise at that moment. The heat needed is the mass flow of water times its specific heat times the rise; dividing by the heater's efficiency gives the input rating to look for. Use the efficiency on the heater's spec sheet: close to 99% for an electric unit; for gas, its thermal efficiency or UEF, typically in the 90s for condensing units and the low 80s for non-condensing ones.
Tankless Sizing Formula
BTU/h = GPM × temperature rise (°F) × 500
kW = L/min × temperature rise (°C) × 0.0696
500 is 8.33 lb per gallon × 60 minutes × 1 BTU per lb per °F (499.7 for water at 20°C, the value the calculator uses). 0.0696 is 998 kg/m³ × 4.1868 kJ/kg·K ÷ 60,000. Divide either result by the efficiency for the input rating.
Worked Example
Two showers at 2.5 GPM and one bathroom faucet at 2.2 GPM running together, 50°F incoming water, heater set to 120°F, 95% efficient:
- • Flow = 2 × 2.5 + 2.2 = 7.20 GPM
- • Temperature rise = 120 − 50 = 70°F
- • Heat delivered = 7.20 × 70 × 499.72 ≈ 251,860 BTU/h (73.8 kW)
- • Heater input = 251,860 ÷ 0.95 = 265,116 BTU/h (77.7 kW)
Showers and faucets blend hot with cold. If the water leaves them at 105°F, the heater only supplies 7.20 × (105 − 50) ÷ (120 − 50) = 5.657 GPM of 120°F water, which needs 197,890 BTU/h delivered and 208,306 BTU/h input. Tick the blending box in the calculator to size this way. Either figure is more than 199,999 BTU/h, the most a single residential gas tankless heater is rated for, so this load needs two units or fewer fixtures running at once.
Shower and Faucet Flow Rates
The fixture flows in the calculator default to the US federal maximums. Water-saving fixtures use less, which lowers the heater size you need.
| Fixture | Federal maximum (10 CFR 430.32) | EPA WaterSense label |
|---|---|---|
| Showerhead | 2.5 GPM (9.5 L/min) at 80 psi | 2.0 GPM or less |
| Bathroom faucet | 2.2 GPM (8.3 L/min) at 60 psi | 1.5 GPM or less |
| Kitchen faucet | 2.2 GPM (8.3 L/min) at 60 psi | No WaterSense specification |
The federal limits apply to fixtures made after 1 January 1994, and some states set lower ones. Older fixtures can use more; time how long one takes to fill a bucket to find its real flow.
Tankless Flow Rate by Temperature Rise
Hot water flow in GPM for a given heat delivered to the water, worked out with the same formula as the calculator. Manufacturers quote a tankless heater's flow at a stated temperature rise, so compare them at your own rise.
| Rise (°F) | 18 kW | 24 kW | 27 kW | 36 kW | 150,000 BTU/h | 199,000 BTU/h |
|---|---|---|---|---|---|---|
| 40 | 3.1 | 4.1 | 4.6 | 6.1 | 7.5 | 10.0 |
| 50 | 2.5 | 3.3 | 3.7 | 4.9 | 6.0 | 8.0 |
| 60 | 2.0 | 2.7 | 3.1 | 4.1 | 5.0 | 6.6 |
| 70 | 1.8 | 2.3 | 2.6 | 3.5 | 4.3 | 5.7 |
| 80 | 1.5 | 2.0 | 2.3 | 3.1 | 3.8 | 5.0 |
Columns are heat delivered to the water. An electric resistance unit delivers practically all of its rated input; for a gas unit multiply the input rating by its efficiency first, so a 199,000 BTU/h unit at 95% delivers about 189,000 BTU/h.
FAQ
Answers to common questions about sizing a tankless water heater.
Frequently Asked Questions
How do I size a tankless water heater?
Add up the flow of every fixture that can run at the same time, find the temperature rise from the incoming water to the outlet setting, and multiply: BTU/h = GPM × rise in °F × 500. Divide by the heater efficiency for the input rating. Two 2.5 GPM showers and a 2.2 GPM faucet with a 70°F rise need about 265,000 BTU/h input at 95% efficiency, more than one residential gas unit (up to 199,999 BTU/h) supplies.
How many GPM does a shower use?
US federal rules (10 CFR 430.32) cap showerheads made since 1994 at 2.5 GPM at 80 psi. WaterSense labelled showerheads use 2.0 GPM or less. Older showerheads can use more; timing how long one takes to fill a bucket gives its actual flow.
What temperature rise should I use?
The difference between the coldest water entering the house and the temperature you set the heater to. Incoming water is coldest in winter, so size for that month; if you are not sure of the figure, measure the cold tap after it has run for a minute.
Why does a tankless heater need so much power?
It heats water only as it flows, so it has to deliver the whole load at once. 7.2 GPM warmed by 70°F takes about 252,000 BTU/h, or 73.8 kW, while the taps are open.
Can an electric tankless heater supply a whole house?
Check the current against your electrical service. The calculator shows the amps at 240 V for the efficiency you enter: the 7.2 GPM, 70°F example needs 73.8 kW delivered, about 311 A at 240 V for an electric unit at 99% efficiency. Electric units suit a smaller flow or a small temperature rise, such as a single fixture or a warm incoming supply.
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