A tankless water heater isn't sized in gallons — it's sized by flow rate and temperature rise. You add up the hot-water GPM of every fixture that could run at once, then figure the temperature rise — your target temperature minus the incoming groundwater temperature. Required output = GPM × 500 × ΔT in BTU/hr. Two showers plus a kitchen faucet is about 4.4 GPM; with 52°F winter inlet water heated to 120°F, that's a 68°F rise and nearly 150,000 BTU/hr. The catch everyone misses: cold inlet water shrinks a unit's output, so buy on the model's GPM-vs-rise curve, never the headline “max GPM.”
Going tankless? The tankless water heater size calculator adds up your simultaneous fixture flow and your temperature rise to give the required BTU/hr (or kW), so you can match it to a real unit — free, no signup. This guide explains why tankless sizing is about flow, not storage, and why the same unit that's plenty in Florida falls short in Minnesota. For the electrical side of an electric unit, pair it with the tankless breaker calculator.
🚿 Flow, not gallons
A tank water heater stores 40 or 50 gallons of pre-heated water, so you size it by storage and recovery. A tankless unit stores nothing — it heats water on demand as it flows through, so the only question that matters is: how much hot water can it heat per minute? That's flow rate, in gallons per minute (GPM).
The way to size it is to picture your peak simultaneous demand — the worst realistic moment when the most hot-water fixtures run at once. Two people showering while the kitchen faucet runs, say. You add up the hot-water GPM of those fixtures, and that sum is the flow the unit has to deliver without the water going cold.
🧮 Adding up simultaneous GPM
Each fixture has a typical hot-water flow (already accounting for the cold that's mixed in at the valve). Common guideline figures: a shower about 1.5 GPM, a kitchen faucet about 1.4, a bathroom faucet about 0.5, a dishwasher or clothes washer about 1.5. You count only what runs at the same time — not every fixture in the house.
Watch the luxury fixtures. Body sprays, rain heads, and big soaker tubs run well above these defaults — a single rain shower can pull 2.5+ GPM by itself. If your bathroom has them, size for them; they're the most common reason a “big enough” tankless unit runs lukewarm.
🌡️ Temperature rise is the hidden variable
This is where tankless sizing surprises people. The unit doesn't just deliver GPM — it delivers GPM at a given temperature rise. Rise (ΔT) is your target temperature minus the temperature of the water coming into the house from the ground:
ΔT = setpoint − incoming groundwater temperature
Groundwater inlet temperature varies enormously by region and season — roughly 40°F in the cold north, 55°F in moderate climates, 68°F in the warm south. Heating to 120°F is a 52°F rise in Florida but an 80°F rise in Minnesota. Because the heater has a fixed BTU output, a bigger rise means fewer gallons per minute — the exact same unit delivers far less hot water in winter than its summer rating.
Then the output math: required BTU/hr = GPM × 500 × ΔT (the 500 comes from 8.34 lb/gal × 60 minutes). For an electric unit, divide BTU/hr by 3,412 to get kilowatts. Residential gas tankless units top out around 199,000 BTU/hr; if your peak demand needs more than that, you're looking at two units in parallel or trimming simultaneous use.
📈 Buy on the curve, not the headline
A tankless unit's advertised “max GPM” is measured at a small, easy temperature rise — great weather, warm inlet water. That number is marketing, not your operating point. Every unit has a GPM-vs-temperature-rise curve, and as the required rise climbs, the deliverable GPM drops.
So sizing ends the same way as a pump: find the point where your peak GPM meets your winter rise, and confirm the unit still delivers it on the manufacturer's curve. A unit that shows 9 GPM at a 35°F rise might only manage 5 GPM at a 70°F rise — and 5 GPM in January is what you actually live with.
⚡ Gas vs electric
Both fuels follow the same GPM-and-rise math; they differ in what the install demands. Gas tankless units hit higher BTU outputs (up to ~199,000 BTU/hr) and generally suit whole-house duty in cold climates, but need proper gas supply and venting. Electric tankless units are simpler to vent but draw enormous current — a whole-house electric unit can pull well over 100 amps at 240 volts.
That electrical load is a real constraint, not a footnote. Before committing to an electric tankless unit, size the circuits and confirm your panel can carry it with the tankless breaker calculator — plenty of electric units are “the right size” thermally but can't be fed by the existing service.
✅ The bottom line
Size a tankless heater by peak simultaneous flow and temperature rise, not gallons: add up the hot-water GPM of fixtures that run at once, subtract your winter inlet temperature from your setpoint to get the rise, and compute GPM × 500 × ΔT for the required BTU/hr. Size for cold-season inlet water and luxury fixtures, confirm the unit delivers your GPM at your rise on its curve, and check the electrical load before buying electric.
Fixture flows and regional inlet temperatures are estimates that vary by product and location, so confirm against the specific model's data and your local water temperature. Start with the tankless sizing calculator to get your required output.
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Free tankless water heater sizing calculator — peak GPM and temperature rise to the flow rate and BTU/kW you need, with WaterSense and Title 20 presets.
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