Tankless water heater sizing calculator
A tankless heater has no gallons to run out of — it is sized by flow. Add the GPM of the fixtures you’ll run at once, set your temperature rise (output minus incoming water), and you have the rating to shop for.
1 Enter your numbers
A tankless is sized by flow at your temperature rise, not by gallons — add the GPM of the fixtures you’ll run at once (about 5.5 GPM here) and note that a colder inlet (a northern winter) cuts a unit’s usable GPM. You need a tankless rated for ≥ 5.5 GPM at a 70 °F rise.
Tankless sizing trips people up because the tank instinct — “how many gallons?” — does not apply. A tankless unit makes hot water on demand, so it can run forever; what it cannot do is exceed its flow rating. Ask two questions instead: how much flow do I draw at once, and how hard does the unit have to work to heat it?
The second question is the temperature rise: the gap between the water you want (usually 120 °F) and the water coming into the house. A Florida inlet near 70 °F is an easy 50-degree lift; a Minnesota winter inlet near 40 °F is an 80-degree lift, and the same unit delivers far fewer usable GPM against it. Sizing by flow and rise is what separates this from the thin single-brand widgets.
Formula
Required flow is the sum of simultaneous fixtures; the rise is output minus inlet:
required_gpm = Σ(fixtures running × fixture_GPM)
ΔT = output_temp − inlet_temp
LABELED fixture flows: shower 2.0 GPM, bath faucet 2.0, kitchen faucet 1.5, dishwasher 1.5, washing machine 2.0. Buy a unit rated for your required GPM at your ΔT — a spec-sheet GPM quoted at a small rise shrinks at a large one.
Worked example
Two showers plus a kitchen faucet, output 120 °F, inlet 50 °F:
2×2.0 + 1×1.5 = 5.5 GPM ΔT = 120 − 50 = 70 °F
You need a whole-house tankless rated for ≥ 5.5 GPM at a 70 °F rise — a class of gas unit, not a point-of-use model. Move that house to a warm-south inlet of 68 °F and the rise drops to 52 °F, so the same 5.5 GPM is easier to hit and a smaller unit may qualify. The inlet is the quiet driver of tankless sizing.
Measure first, avoid a wrong size
Size for the rise you actually face. A manufacturer’s headline GPM is often quoted at a gentle 45-degree rise; at a northern 80-degree rise the same box may only manage half that flow.
- Count only true overlaps. If nobody runs the kitchen tap during a shower, do not stack it — oversizing a tankless wastes money and can short-cycle.
- Winter inlet is the worst case. Size to your coldest-month inlet temperature (see the region table), not the summer figure, or the unit will fall short exactly when you need it.
- Gas and electric differ hard here. High-GPM whole-house flow at a big rise almost always means gas; electric tankless suits point-of-use or warm-climate duty.
- Confirm the label. Match your required GPM to the unit’s rated flow at your rise on its spec sheet.
Reference table
LABELED fixture flow (GPM), used to weight simultaneous draws.
| Fixture | Flow (GPM) |
|---|---|
| Shower | 2.0 |
| Bath faucet | 2.0 |
| Kitchen faucet | 1.5 |
| Dishwasher | 1.5 |
| Washing machine | 2.0 |
LABELED incoming water temperature by US region — a colder inlet means a bigger rise and a bigger unit.
| Region | Inlet temp (°F) | Rise to 120 °F |
|---|---|---|
| Cold / northern | 40–50 | ≈ 75 °F |
| Temperate / middle | 50–60 | ≈ 65 °F |
| Warm / southern | 60–77 | ≈ 52 °F |