Tankless Water Heater GPM Chart: Flow by Fixture and Home
A shower uses about 2 GPM, a kitchen faucet 1.5 GPM, so the two together need 3.5 GPM, or 122,500 BTU/hr of heating with 50 °F groundwater heated to 120 °F. Add the fixtures you run at the same time in the tankless water heater calculator to size your own.
Part of the project planner: Planning pools, ponds and water storage.
Flow and heating power by fixture
Typical flow rates. The BTU and gas input are for that fixture alone with 50 °F incoming water heated to 120 °F:
| Fixture | Flow | Heat needed (BTU/hr) | Gas input at 90% (BTU/hr) |
|---|---|---|---|
| Bathroom faucet | 1.0 GPM | 35,000 | 38,889 |
| Kitchen faucet | 1.5 GPM | 52,500 | 58,333 |
| Dishwasher | 1.5 GPM | 52,500 | 58,333 |
| Shower | 2.0 GPM | 70,000 | 77,778 |
| Washing machine | 2.0 GPM | 70,000 | 77,778 |
| Bathtub filling | 4.0 GPM | 140,000 | 155,556 |
What a typical household needs
Same temperatures. The last column assumes the largest common gas unit has an input around 199,000 BTU/hr:
| Fixtures at the same time | Flow | Heat needed (BTU/hr) | Gas input at 90% (BTU/hr) | Size |
|---|---|---|---|---|
| 1 shower and the kitchen faucet | 3.5 GPM | 122,500 | 136,111 | One gas unit |
| 2 showers at once | 4.0 GPM | 140,000 | 155,556 | One gas unit |
| 2 showers and the dishwasher | 5.5 GPM | 192,500 | 213,889 | Two gas units or a bigger heater |
| 2 showers, kitchen faucet and washer | 7.5 GPM | 262,500 | 291,667 | Two gas units or a bigger heater |
| 3 showers and the dishwasher | 7.5 GPM | 262,500 | 291,667 | Two gas units or a bigger heater |
How cold groundwater changes the size
Two showers and the dishwasher (5.5 GPM) heated to 120 °F, as the incoming water gets colder:
| Incoming water | Temperature rise | Heat needed (BTU/hr) | Gas input (BTU/hr) | Electric power | Current at 240 V |
|---|---|---|---|---|---|
| 40 °F | 80 °F | 220,000 | 244,444 | 64.5 kW | 269 A |
| 50 °F | 70 °F | 192,500 | 213,889 | 56.4 kW | 235 A |
| 60 °F | 60 °F | 165,000 | 183,333 | 48.4 kW | 201 A |
| 70 °F | 50 °F | 137,500 | 152,778 | 40.3 kW | 168 A |
Size for your coldest month. A northern winter can need about 14% more heating than the 50 °F case, which is why the same unit can feel strong in summer and weak in January.
Ways to need a smaller heater
- Fit low-flow shower heads: a 1.5 GPM head instead of a 2.0 GPM one trims the peak.
- Stagger the dishwasher and washer so they do not run during morning showers.
- Use a second small unit or a point-of-use heater for a far-off bathroom.
Gas versus electric, venting and the installation are covered in tankless water heater sizing. If you are weighing a tank instead, see what size water heater you need and the water heater size calculator. An electric unit needs a large circuit: check it against the generator size calculator before a backup plan.
Fixture flow rates are typical: check yours. Gas and electric water heaters should be installed by a licensed plumber or electrician.
Frequently asked questions
What size tankless water heater do I need for a family of four?
Two showers running together with the dishwasher come to 5.5 GPM. With 50 °F incoming water that needs 192,500 BTU/hr of heat, which needs a gas input near 214,000 BTU/hr. That is more than one 199,000 BTU/hr unit supplies, so plan on two units or low-flow fixtures.
How many GPM does a shower use?
About 2.0 GPM in this chart. Federal rules cap new shower heads at 2.5 GPM, and low-flow ones use 1.5 to 1.8 GPM.
What is GPM on a tankless water heater?
Gallons per minute: how much hot water the unit can heat to your target temperature at once. The number printed on a tankless heater depends on the temperature rise, so it falls as the incoming water gets colder.
Why does my groundwater temperature matter?
The colder the incoming water, the more heat each gallon needs. The same two showers and dishwasher need 192,500 BTU/hr at 50 °F, but 220,000 BTU/hr at 40 °F.
Can an electric tankless heater run a whole house?
Rarely. 5.5 GPM at 50 °F groundwater needs 56.4 kW, about 235 amps at 240 V, more than many homes can spare. Gas units or a point-of-use heater for one bathroom are more common.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.