Tankless Water Heater Sizing
Size a tankless (on-demand) water heater from the flow you want to run at once and the temperature rise — get the required gas BTU/hr or electric kW.
Calculator
To raise 4.0 GPM by 45 °F a tankless heater needs about 90,000 BTU/hr (gas) or 26.4 kW (electric). Add up the fixtures you want to run at once, and use your coldest incoming water temp — winter ground water is colder and raises the required size.
A tankless heater never stores hot water, so it is not sized by gallons — it is sized by how much water you want to heat per minute and by how far it has to raise the temperature. Undersize it and the flow drops or the water turns lukewarm when two fixtures run at once.
Formula
First find the temperature rise, then the energy needed to heat that flow:
ΔT (°F) = target temp − incoming temp
gas load (BTU/hr) = flow (GPM) × 500 × ΔT
electric load (kW) = flow (GPM) × ΔT × 0.1465
The 500 factor is 60 min/hr × 8.34 lb/gal × 1 BTU/lb·°F; the 0.1465 factor is the same physics expressed in kilowatts. Incoming water is coldest in winter — always size on your coldest groundwater temperature so the unit keeps up year-round.
Worked example
You want to run 4 GPM (say a shower plus a kitchen faucet) and your winter groundwater is 55 °F heated to 100 °F — a 45 °F rise.
gas = 4 × 500 × 45 = 90,000 BTU/hr
electric = 4 × 45 × 0.1465 = 26.4 kW
So a gas unit around 90,000 BTU/hr (or a 26.4 kW electric) covers that draw. Electric whole-house tankless at 26 kW needs a large dedicated circuit — check the panel and service size.
Background & practice
The single biggest sizing mistake is using an average incoming temperature instead of the coldest. In the northern US, winter groundwater can sit near 40 °F, which turns a 45 °F rise into a 65 °F rise and roughly doubles the flow-limited output you feel. Decide how many fixtures truly run at once — most households never need more than two — and size to that, not to every fixture in the house.
Flow rates to add up: a modern shower is about 2.0 GPM, a bathroom faucet 1.5 GPM, a kitchen faucet 1.5–2.2 GPM. Measure your own with the bucket-test flow tool. For a storage-tank alternative, size on peak-hour demand instead.
Reference table
| Flow ↓ / rise → | 40 °F | 60 °F | 80 °F |
|---|---|---|---|
| 2.0 GPM | 40,000 BTU/hr | 60,000 BTU/hr | 80,000 BTU/hr |
| 4.0 GPM | 80,000 BTU/hr | 120,000 BTU/hr | 160,000 BTU/hr |
| 6.0 GPM | 120,000 BTU/hr | 180,000 BTU/hr | 240,000 BTU/hr |
Gas BTU/hr required (flow × 500 × ΔT). Divide by ~1000 for CFH of natural gas; for electric, multiply the GPM × ΔT by 0.1465 for kW.
Frequently asked questions
What size tankless water heater do I need for 4 GPM at a 45 °F rise?
About 90,000 BTU/hr for gas, or roughly 26 kW for electric. That covers a shower plus a faucet running together with a 45 °F rise. Colder incoming water needs more.
Why does incoming water temperature matter so much?
Because the heater is sized on the rise, not the target. The same 105 °F output needs far more energy from 40 °F winter water than from 70 °F summer water. Always size on your coldest groundwater temperature.
How many fixtures should I size for?
Size for the fixtures that realistically run at the same time — usually one shower plus one sink. Add each fixture’s GPM; you can measure real flow with the bucket test.
Can my electrical service handle an electric tankless?
A whole-house electric tankless can draw 26 kW or more, needing multiple large circuits and often a 200 A service. Gas tankless avoids that but needs adequate gas supply and venting. Confirm both with a licensed pro.