Free standard shipping on orders over $149.97
Our tankless water heater size calculator turns the fixtures you use at the same time and your climate into the flow rate, temperature rise, gas BTU and electric kW you need.
The calculator uses standard formulas: gas input BTU/h = GPM × 500 × temperature rise ÷ efficiency (0.95 condensing, 0.82 non-condensing), and electric kW = GPM × temperature rise × 0.1465. It is an estimate. Check the product page and manual, and ask your installer to confirm.
This is the core of how to size a tankless water heater: you calculate size of tankless water heater needs from flow and temperature rise, not tank gallons.
Tankless water heaters are sized by gallons per minute (GPM) at peak demand, not by gallons stored. List the hot-water fixtures your household realistically uses at the same time.
| Fixture | Typical hot water flow |
|---|---|
| Shower (federal max 2.5 GPM) | 1.5–2.5 GPM |
| Bathroom faucet | 0.5–1.5 GPM |
| Kitchen faucet | 1.5–2.2 GPM |
| Dishwasher | 1.0–1.5 GPM |
| Washing machine (warm) | 1.5–2.0 GPM |
| Bathtub filling | 4.0+ GPM |
Hot water is mixed with cold at the tap, so a 2.0 GPM shower usually draws less than 2.0 GPM of hot water. The calculator uses full fixture flow as a safety margin.
Temperature rise = the temperature you want (usually 105–120°F) minus the temperature of the water entering your home in winter. Groundwater in the northern U.S. can be near 40°F in winter; along the Gulf Coast and in Florida it can be 65–75°F. That difference changes how much a unit can deliver.
| Region | Winter inlet water | Rise to 120°F |
|---|---|---|
| Northern states | 37–45°F | 75–83°F |
| Central states | 47–55°F | 65–73°F |
| Southern states | 57–65°F | 55–63°F |
| Florida, south Texas, Arizona low desert | 65–75°F | 45–55°F |
Manufacturers publish a flow chart showing GPM at different rises. A unit marketed as "11 GPM" may deliver that only at about a 35°F rise. Use the flow at your rise.
| BTU input (condensing) | GPM at 45°F rise | GPM at 60°F rise | GPM at 75°F rise |
|---|---|---|---|
| 120,000 | 5.1 | 3.8 | 3.0 |
| 150,000 | 6.3 | 4.8 | 3.8 |
| 180,000 | 7.6 | 5.7 | 4.6 |
| 199,000 | 8.4 | 6.3 | 5.0 |
| Electric kW | GPM at 45°F rise | GPM at 60°F rise | GPM at 75°F rise | Approx. amps at 240V |
|---|---|---|---|---|
| 13 kW | 2.0 | 1.5 | 1.2 | 54 A |
| 18 kW | 2.7 | 2.0 | 1.6 | 75 A |
| 27 kW | 4.1 | 3.1 | 2.5 | 113 A |
| 36 kW | 5.5 | 4.1 | 3.3 | 150 A |
Values are calculated, rounded and assume 95% efficiency for gas. Real-world performance varies by model.
An 8 GPM tankless water heater or 10 GPM tankless water heater rating is a maximum; use the tankless water heater size chart below to see what you’ll actually get. If you were comparing against a 10 gallon electric water heater mini tank, note that a mini tank serves one fixture briefly, while these units serve the whole house.
Still deciding on fuel? Read gas vs electric tankless water heaters.
Filter whole-house units by the GPM and fuel the calculator recommends. Or send us your results and we’ll double-check them.
Add up the GPM of the fixtures you use at once (for example, two showers at 2 GPM plus a faucet at 1 GPM = 5 GPM), then find a unit that delivers that flow at your winter temperature rise. The calculator above does it for you.
Usually 5–7 GPM at your temperature rise. That is typically a 180,000–199,000 BTU gas or propane unit, or a 27–36 kW electric unit if you live in a warm climate.
A unit rated 10–11 GPM is usually rated at a small temperature rise. At a 70°F rise in a cold climate, the same unit typically delivers about 5–5.5 GPM, enough for two showers and a faucet.
No, the heater does not boost water pressure. Flow is limited by its heating capacity. If you need more hot water flow, choose a larger unit, link two units, or reduce simultaneous demand with low-flow fixtures.
There is no direct gallon equivalent because tankless units never run out. For a home that used a 50-gallon tank, a 150,000–199,000 BTU gas unit (about 5–8 GPM at typical rises) is the usual replacement.
This guide is general information for U.S. homeowners, not professional advice. Gas, electrical and venting work must follow the manufacturer’s instructions and local codes, and in most areas requires a licensed plumber, electrician or gas fitter and a permit. Specifications and prices change; always confirm details on the product page and in the installation manual.
