01What this calculator tells you
A tankless (demand) water heater has no storage tank — it heats water only as it flows, so it must produce your entire peak demand in real time. Sizing it therefore comes down to two numbers: the peak flow rate in gallons per minute you need at once, and the temperature rise between your incoming water and your target temperature. This calculator works out both and converts them into the heat capacity you need.
Enter how many of each fixture might run simultaneously, pick your region so the calculator knows your incoming water temperature, set your desired output temperature, and choose gas or electric. You get the peak GPM, the temperature rise, the required heat output in BTU/hr, and the recommended capacity — a BTU/hr input rating for gas or a kW figure for electric. Tankless units are among the most efficient ways to make hot water; ENERGY STAR qualifies models that cut standby losses a storage tank cannot avoid.
02Fixture flow rates
Peak demand is simply the sum of the flow rates of the fixtures you expect to run at the same time. The calculator uses the modern, low-flow values below; older fixtures can run higher, so bump the count if yours are dated. Efficient showerheads are capped at 2.0 GPM under EPA WaterSense, and labeled bathroom faucets at 1.5 GPM per the WaterSense faucet spec.
03Temperature rise and your region
The second half of sizing is the temperature rise: your target output temperature minus the temperature of the water entering the house. Incoming (groundwater) temperature swings by geography and season — from around 37°F in the far north to 77°F on the Gulf Coast — and it is coldest in the coldest month. A unit is only correctly sized if it delivers your peak GPM at the winter rise, which is why the region selector uses cold-season values rather than annual averages.
The same unit delivers noticeably more GPM in Florida than in Minnesota, because a smaller rise takes less energy per gallon. That is the single biggest reason a tankless model rated “up to 9 GPM” can disappoint in a cold climate.
- For each fixture, enter how many might run at the same time during a busy morning or evening.
- Pick your region so the calculator uses your coldest-month incoming water temperature.
- Set your desired output temperature (105–120°F is typical for a whole house) and choose gas or electric.
- Press Calculate to see peak GPM, temperature rise, heat output, and the recommended capacity.
Hot water is one of the largest slices of home energy use — the U.S. Energy Information Administration puts water heating near the top after space conditioning — so sizing correctly saves both up-front and running cost. Planning your daily hot-water use as well? Our water intake calculator is a companion tool for the drinking side of the equation.
This is a sizing estimate, not an installation design. Confirm the final model and its gas or electrical requirements with the manufacturer’s spec sheet and a licensed installer.
- Flow rates are typical, not exact. Your fixtures may differ; a rain shower or an unrestricted tub spout raises peak demand.
- Gas sizing assumes ~82% thermal efficiency. A high-efficiency condensing unit needs a lower input rating for the same output; a non-condensing unit needs more.
- Electric units need serious power. A whole-house electric tankless can draw 100–150 amps, often requiring multiple double-pole breakers and heavy feeders — size the wiring with our conduit fill calculator and confirm your panel has capacity.
- Fuel choice affects running cost. Gas is usually cheaper to run where available; compare lifetime energy cost the way you would any home upgrade, much like our energy efficiency upgrade calculator does for home retrofits.
- Minimum activation flow. Most tankless units need roughly 0.5 GPM to fire, so a trickle at a faucet may not trigger heating.
04Related calculators
Working through a related project? Try our Heat Pump Sizing Calculator, U-Value Calculator, and Net Zero Home Calculator.
01The sizing formula
Sizing a tankless heater is a heat-balance problem: how much energy per hour it takes to raise your peak flow of water by your temperature rise. Two inputs drive everything — peak flow and rise — and one constant ties them to energy.
Where:
- GPM= peak simultaneous flow in gallons per minute.
- ΔT= temperature rise in °F (output minus incoming).
- Q= heat added to the water, in BTU per hour.
- efficiency= the unit’s thermal efficiency (≈0.82 non-condensing, up to ~0.95 condensing).
02Worked example
Take a home where, at peak, one shower (2.0 GPM) and the kitchen faucet (1.5 GPM) run together, incoming water is 50°F in winter, and the target is 110°F. Work it one line at a time:
Move that same home to the far north (37°F incoming) and the rise jumps to 73°F, pushing Q to about 128,000 BTU/hr and the gas input past 155,000 — the same fixtures, a meaningfully bigger unit. That climate sensitivity is exactly why the region selector matters.