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Tankless Water Heater Size Calculator: GPM, Temperature Rise and BTU
A tankless heater is sized by peak flow and temperature rise, not by tank gallons. Add the flow of fixtures used at the same time, subtract the coldest incoming water temperature from the delivery temperature, and use BTU/h = 500 × GPM × rise. Two 2.0 GPM showers with 50 °F inlet water need about 110,000 BTU/h of output, roughly 116,000 BTU/h of gas input or about 33 kW electric.
Tankless size calculator
The sizing formula
Heating water takes 1 BTU per pound per °F. A gallon of water weighs about 8.33 lb, and a gallon per minute is 60 gallons an hour, so one GPM raised one degree needs about 500 BTU/h.
Flow (GPM) = Showers × Shower GPM + Faucets × 1.5 + Appliances
Rise (°F) = Delivered temp − Inlet temp
Output (BTU/h) = 500 × GPM × Rise
Gas input = Output ÷ Efficiency · Electric kW = Output ÷ 3,412 ÷ 0.99
Electric units convert nearly all their power to heat, so 0.99 is used for them. Manufacturers publish the same relationship as a flow chart: the maximum GPM their unit can deliver at each temperature rise.
Worked example
Two showers at 2.0 GPM running together, no faucets, winter inlet water at 50 °F, shower mixed to 105 °F, condensing gas unit:
- Flow = 2 × 2.0 = 4.0 GPM
- Rise = 105 − 50 = 55 °F
- Output = 500 × 4.0 × 55 = 110,000 BTU/h
- Gas input = 110,000 ÷ 0.95 ≈ 116,000 BTU/h
- Electric = 110,000 ÷ 3,412 ÷ 0.99 ≈ 32.6 kW
Most whole-house gas models are rated 150,000–199,000 BTU/h input, so a 160,000–180,000 BTU/h unit covers this with margin for a sink running at the same time. The electric figure is near the top of what residential electric tankless units offer (about 27–36 kW), and 32.6 kW draws roughly 136 A at 240 V. That usually means a dedicated set of breakers and possibly a service upgrade.
Inlet water temperatures across the US
Groundwater temperature drives the temperature rise, and it is lowest in late winter. Approximate winter inlet temperatures:
| Region | Typical winter inlet | Rise to 105 °F | Rise to 120 °F |
|---|---|---|---|
| Northern tier (MN, WI, ME, MT) | 35–45 °F | 60–70 °F | 75–85 °F |
| Central (OH, PA, CO, MO) | 45–55 °F | 50–60 °F | 65–75 °F |
| South (GA, TX, southern CA) | 55–65 °F | 40–50 °F | 55–65 °F |
| Gulf Coast, FL, desert Southwest | 65–75 °F | 30–40 °F | 45–55 °F |
To measure yours, run a cold tap for a few minutes in February and check it with a kitchen thermometer.
Typical fixture flow rates
- Shower head: 1.5–2.5 GPM (WaterSense ≤ 2.0)
- Bathroom faucet: 0.5–1.5 GPM
- Kitchen faucet: 1.5–2.2 GPM
- Bathtub filler: 4–6 GPM (a tub alone can exceed what one tankless unit delivers in cold climates)
- Dishwasher: about 1–1.5 GPM while filling
- Clothes washer on warm: about 1.5–2 GPM while filling
Using 105 °F for showers is realistic because the mixing valve blends in cold water. If you set the heater to 120 °F and want the dishwasher or a tub fill at full temperature, run the calculator at 120 °F as well and size for the larger of the two results.
Gas line, venting and electrical limits
A 199,000 BTU/h gas tankless needs roughly five times the gas flow of a typical 40,000 BTU/h tank heater. Existing 1/2-inch gas lines often cannot supply it, and the installer must check the meter capacity and pipe sizing for the whole house. Condensing units vent through PVC and need a condensate drain; non-condensing units need stainless steel venting. Electric models need heavy dedicated circuits.
Gas piping, venting and high-amperage electrical work must be done by licensed professionals and inspected. An undersized gas line or wrong vent material can cause incomplete combustion and carbon monoxide.
Electric tankless amperage
Electric tankless heaters draw a lot of current at once, because they heat water on the fly rather than over an hour. Divide watts by the voltage to get amps:
| Unit size | Current at 240 V |
|---|---|
| 11 kW | about 46 A |
| 18 kW | 75 A |
| 24 kW | 100 A |
| 27 kW | about 113 A |
| 36 kW | 150 A |
Larger units split this load across two to four separate circuits, each with its own breaker; the installation manual specifies the breaker and wire sizes. A house with a 100 A or 150 A service usually cannot add a 27–36 kW unit without an upgrade. A 200 A service often can, after a load calculation. Point-of-use units of 3–11 kW under a single sink are far easier to supply.
Is tankless the right choice?
If the flow and gas or electrical requirements look large, compare with a storage tank sized by first-hour rating using the water heater sizing calculator. The tankless vs tank comparison covers cost, lifespan and the cold-water sandwich effect. If hot water takes a long time to reach far fixtures, a recirculation pump may matter more than the heater itself. Gas and electric operating costs can be compared with the fuel cost calculator by entering your water-heating load.
Frequently asked questions
What size tankless water heater do I need for a family of four?
Usually one that can supply two showers at once: about 4–5 GPM. In northern climates that means a gas unit around 180,000–199,000 BTU/h input; in the South, 150,000–160,000 is often enough. Whole-house electric units of 27–36 kW work best where inlet water is warm.
How many GPM does a tankless water heater need?
Add the flow of everything that runs at the same time at peak. Two showers and a faucet is about 5–5.5 GPM with standard fixtures.
Why does my tankless heater go lukewarm when two showers run?
The demand has exceeded its capacity at your temperature rise, which is most common in winter. Low-flow shower heads, a lower setpoint or a larger or second unit are the usual fixes.
Last reviewed 2026-10-28