Tankless Water Heater Calculator

Size a tankless water heater from the fixtures you run at once and your temperature rise (GPM + BTU).

Inputs
Showers running1
Sink faucets running1
Tub fillers running0
Dishwasher / washer running0
°F
°F
Results update as you type — no submit
ResultsLIVE
Required flow
3.25 GPM
≈ 12.3 L/min
Temperature rise70 °F · 38.9 °C
Heating demand113,750 BTU/hr · 33.3 kW
Pick a tankless unit rated for at least this GPM at this temperature rise — a unit’s maximum flow drops as the rise climbs. An installer confirms gas, venting, and model.
Formula
flow = 3.25 GPM × 500 × 70°F rise = 113,750 BTU/hr
How it works

A tankless (demand) water heater makes hot water on the fly, so unlike a tank you size it by flow rate, not gallons stored. The US Department of Energy method has two steps: add up the flow of every fixture you want to run at the same time to get peak GPM, then find your temperature rise — the desired hot-water temperature minus the temperature of the water coming into the house.

Typical fixture flows are about `2.5` GPM for a shower, `0.75` for a bathroom faucet, `4.0` for a tub filler, and `2.0` for a dishwasher or clothes washer. Running one shower and one faucet at once is `3.25` GPM. If your groundwater arrives at `50°F` and you want `120°F` at the tap, the rise is `70°F`. A tankless unit must deliver your peak GPM at that rise — and every unit's maximum flow drops as the required rise climbs, which is why the same heater serves more showers in Florida than in Minnesota.

The calculator also reports the heating power the job demands, using the water-heating identity BTU/hr = GPM × `500` × rise. At `3.25` GPM and a `70°F` rise that is about `113,750` BTU/hr — a large gas unit. Use these numbers to compare models by their rated GPM at your rise; a licensed installer confirms gas line, venting, and the final model.

Worked example
`1` shower (`2.5`) + `1` faucet (`0.75`) = `3.25` GPM · incoming `50°F` → target `120°F`
temperature rise = 120 − 50 = `70°F`
heating demand = 3.25 × 500 × 70 = `113,750 BTU/hr` (`33.3 kW`)
Questions about tankless sizing
Add the flow of the fixtures you will run together for peak GPM, then size a unit that delivers that GPM at your temperature rise. Two showers at once is about 5 GPM; if your rise is 70°F you need a unit rated for 5 GPM at 70°F, which is a large whole-house gas model.
SOURCES — US DOE / energy.gov demand water-heater sizing · water-heating power identity (GPM × 500 × °F)
Rule-of-thumb estimate — a Manual J load calculation by a licensed HVAC pro is definitive
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