Size an on-demand water heater the way installers do: add up the flow of every fixture that may run at the same time, then find the temperature rise from your incoming water to the hot water temperature you want. The calculator turns that into the gas BTU/h input or electric kW (and amps) the heater must deliver.
How to use the Tankless Water Heater Size Calculator
Count the fixtures that could run at the same time on a busy morning, for example two showers and a kitchen faucet.
Check the flow rates. The defaults are typical modern fixtures; change them if you know your showerhead or faucet ratings.
Enter your coldest incoming water temperature (the winter groundwater temperature) and the hot water temperature you want, usually 120 °F (49 °C).
Pick gas or electric and the efficiency of the unit type you are considering.
Read the required GPM and the BTU/h or kW size, then compare it with the flow-vs-rise chart in a heater's spec sheet.
The tankless sizing formula
A tankless heater must heat the full flow instantly, so its size depends on flow and temperature rise, not on household size:
BTU/h output = GPM × 500 × ΔT (°F)
GPM: the sum of the flow rates of all fixtures running at the same time
ΔT: desired hot water temperature − incoming water temperature
500: 8.33 lb per gallon of water × 60 minutes per hour (1 BTU raises 1 lb of water by 1 °F)
Gas input = output ÷ efficiency. Electric kW = output ÷ 3,412 ÷ efficiency, and amps at 240 V = watts ÷ 240.
Example: two 2.0 GPM showers and a 1.5 GPM kitchen faucet = 5.5 GPM. Raising 50 °F water to 120 °F is a 70 °F rise: 5.5 × 500 × 70 = 192,500 BTU/h of output, or about 202,600 BTU/h input at 95% efficiency. That is more than the 199,000 BTU/h most residential gas units offer, so you would either use 1.75 GPM showerheads or install two units.
Typical fixture flow rates
Fixture
Typical GPM
L/min
Notes
Shower
1.5 – 2.5
5.7 – 9.5
2.5 GPM is the US federal maximum; WaterSense is 2.0 or less
Bathroom faucet
0.5 – 1.5
1.9 – 5.7
WaterSense lavatory faucets are 1.5 GPM or less
Kitchen faucet
1.5 – 2.2
5.7 – 8.3
2.2 GPM is the common US maximum
Bathtub
4 – 7
15 – 26
Tub spouts have no flow restrictor
Dishwasher
1 – 2
3.8 – 7.6
Fills briefly, often heats its own water
Washing machine
1.5 – 3
5.7 – 11.4
Only the hot share on warm/hot cycles
Flow a heater can deliver by temperature rise
Maximum GPM = output BTU/h ÷ (500 × rise). Values for a 199,000 BTU/h gas unit at 95% efficiency and a 27 kW electric unit at 99%.
Rise
199,000 BTU/h gas
27 kW electric
35 °F
10.8 GPM
5.2 GPM
45 °F
8.4 GPM
4.0 GPM
55 °F
6.9 GPM
3.3 GPM
65 °F
5.8 GPM
2.8 GPM
75 °F
5.0 GPM
2.4 GPM
85 °F
4.4 GPM
2.1 GPM
Frequently asked questions
How do I size a tankless water heater?
You need two numbers: the peak flow rate and the temperature rise. Add the gallons per minute of every fixture that may run at once, then subtract the incoming water temperature from the desired output temperature. A heater is big enough when its rated flow at that temperature rise is at least your peak flow.
What is the formula for tankless water heater BTU?
Heat output in BTU per hour = GPM × 500 × temperature rise (°F). The 500 comes from 8.33 pounds per gallon × 60 minutes. Divide by the heater's efficiency to get the gas input rating, or divide the output by 3,412 to get kilowatts for an electric unit. For example, 5 GPM with a 70 °F rise needs 175,000 BTU/h of output, about 184,000 BTU/h input at 95% efficiency.
What incoming water temperature should I use?
Use the coldest groundwater temperature of the year, since that is when the heater works hardest. In the US it ranges from about 40 °F in northern states to about 70 °F in the far south. If you are not sure, 50 °F is a reasonable middle value; colder climates should use 40 to 45 °F.
Is an electric tankless water heater big enough for a whole house?
Often only in warm climates. An electric unit needs about 1 kW for every 0.68 GPM of flow per 10 °F of rise, so a whole-house 36 kW model gives roughly 3.5 GPM at a 70 °F rise but nearer 5 GPM at a 50 °F rise. Large electric units also need 150 A or more of capacity, which many homes do not have spare.
How many GPM does a shower use?
US federal rules limit new showerheads to 2.5 GPM, and many WaterSense models use 2.0 GPM or less. In practice a shower draws a mix of hot and cold water, so sizing with the full showerhead flow at the output temperature adds a safety margin.
Should I round up the size?
Yes. Pick a model whose rated flow at your temperature rise is at least the calculated GPM. Undersized tankless heaters cannot fail gracefully: the water gets cooler as flow goes up. A small margin also covers efficiency loss from scale build-up.
Is my information stored or uploaded?
No. The calculation happens in your browser. Your last inputs are only remembered locally so they reappear when you return.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.