Formula
A tankless heater is sized by the water that has to be hot at the same moment, and by how many degrees that water has to climb. Add count times gpm for each fixture. Temperature rise is outlet minus inlet. The heat that has to enter the water is 500 × gpm × °F. The 500 is 8.33 pounds per gallon times 60 minutes per hour times 1 BTU per pound per °F, rounded. That is the standard rule of thumb. It is not a Uniform Energy Factor.
Q = Σ (count × gpm)
Output BTU/h = 500 × Q × ΔT
Gas input = output / efficiency
kW = output / 3,412.14 / efficiency
Amps = kW × 1,000 / volts
Gas efficiency defaults to 0.95, a condensing thermal efficiency, with 0.82 as the usual non-condensing figure. Electric defaults to 0.99. Type the number from the spec sheet. Input above 199,900 BTU/h is flagged because that is a typical maximum for one residential gas tankless unit. The flag suggests two units or less simultaneous demand. It does not size the gas pipe. Reverse mode rearranges the same output formula: maximum gpm = input × efficiency / (500 × rise). An electric reverse entry starts as kilowatts and is multiplied by 3,412.14 to reach BTU/h.
Inlet has to be from 32 to 90 °F. Rise has to be greater than 0 and at most 120 °F. Flows and counts cannot be negative. A bad input stops the result and says why. Winter inlet is the one that matters, because groundwater and municipal water are colder then, and a unit sized on a summer inlet will fall short in January. 120 °F outlet follows the common CPSC and DOE scald-safety guidance. Dishwashers that want hotter water are a separate rise, and the appliance sheet should say so. Low-flow showerheads and faucets, WaterSense at 2.0 and 1.5 gpm against federal maximums of 2.5 and 2.2, are how a house stays inside one heater. Storage heaters are a different problem: first-hour rating, not this flow-and-rise pair.
Electric amps are information only. Breaker and conductor sizing follow the manufacturer’s installation instructions and NFPA 70 (NEC) 2023, Article 422. The wire size calculator and the amp calculator are context, not a recommendation from this result. The pipe volume and friction loss pages describe the water line. The well pump page is the supply side, if the house is on a well.
Worked examples
Gas, two showers and a bathroom faucet. 2 × 2.0 + 1 × 1.5 = 5.5 gpm. Inlet 50 °F, outlet 120 °F, rise 70 °F. Output = 500 × 5.5 × 70 = 192,500 BTU/h. Input at 0.95 = 202,632 BTU/h, which is over 199,900 BTU/h, so the flag is on.
Electric, one shower. 2.0 gpm, inlet 60 °F, outlet 120 °F, rise 60 °F. Output = 60,000 BTU/h. Input = 60,000 / 0.99 / 3,412.14 = 17.76 kW. At 240 V that is 74.0 A. At 208 V it is 85.4 A. Neither number is a breaker.
Reverse. A gas unit with 199,900 BTU/h input at 0.95 delivers 189,905 BTU/h to the water. At a 70 °F rise, 189,905 / 35,000 = 5.43 gpm.
Reference
Fixture defaults are typical published maximums, not a survey of your house. Edit them.
| Fixture | Default gpm | Why that number |
|---|---|---|
| Shower | 2.0 | WaterSense showerheads are 2.0 gpm maximum. The federal maximum is 2.5 gpm at 80 psi. Use 2.5 for an older head. |
| Bathroom faucet | 1.5 | WaterSense lavatory faucets are 1.5 gpm maximum. The federal maximum is 2.2 gpm. |
| Kitchen faucet | 2.2 | Federal maximum for a kitchen faucet is 2.2 gpm. |
| Bathtub fill | 4.0 | Typical fill rate. Tubs vary widely. |
| Dishwasher | 1.5 | Estimate. The appliance spec sheet wins. |
| Clothes washer, hot fill | 2.0 | Estimate for a hot fill. The appliance spec sheet wins. |
Questions
What size tankless water heater do I need?
Two numbers size it: the gallons per minute of the fixtures that run at the same time, and the temperature rise from winter inlet water to the outlet you want. Output BTU/h is 500 × gpm × rise. Divide by the unit’s thermal efficiency for the gas input, or by 3,412.14 and the efficiency for electric kilowatts. Then shop a unit rated at least that flow at that rise. This is a sizing estimate. The spec sheet and a licensed installer decide the model.
How many GPM does a shower use?
A WaterSense-labeled showerhead is 2.0 gpm at most. The federal maximum is 2.5 gpm at 80 psi, which is the right number for many older heads. A WaterSense bathroom faucet is 1.5 gpm maximum, against a federal maximum of 2.2 gpm. A kitchen faucet’s federal maximum is 2.2 gpm. Bathtub fill, dishwashers, and clothes washers vary. The numbers on this page are typical and you can edit them. Low-flow fixtures are the cheapest way to shrink the heater.
What temperature rise should I use?
Rise is the outlet you want minus the coldest inlet you actually get. This page starts at 50 °F inlet and 120 °F outlet, a 70 °F rise. 120 °F is the common scald-safety outlet from CPSC and DOE guidance. Groundwater is colder in the north and in winter. Size for the winter inlet. There is no state-by-state map on this page. If you know the winter inlet, type it.
Can one tankless unit run two showers?
Two showers at 2.0 gpm plus a bathroom faucet at 1.5 gpm is 5.5 gpm. From 50 °F to 120 °F that is 192,500 BTU/h out of the water, and about 202,600 BTU/h into a 95% condensing unit. That is over 199,900 BTU/h, a typical ceiling for one residential gas tankless heater. The page flags it. Two heaters, or not running both showers with the faucet, is the usual way through. A point-of-use heater can take one distant fixture off the whole-house unit. A storage heater is sized by first-hour rating instead, which is a different calculator.
How many amps does an electric tankless heater need?
The arithmetic is on the result: kilowatts times 1,000, divided by the supply voltage. One 2.0 gpm shower from 60 °F to 120 °F is 60,000 BTU/h, about 17.76 kW at 99% efficiency, 74.0 A at 240 V and 85.4 A at 208 V. Those amps are information only. Breaker and wire sizing follow the manufacturer’s installation instructions and NEC Article 422 (NFPA 70, 2023). This page does not pick either. The wire size calculator is there for context once an electrician has the nameplate.
Gas or electric tankless?
Gas input is the output divided by thermal efficiency. Condensing units are often around 0.95. Non-condensing units are often around 0.82. Use the spec sheet, not the UEF. Gas also needs a supply pipe and a vent that this page does not size. Electric input is smaller in the utility closet and large on the service. Efficiency here defaults to 0.99. Either way, a unit that is too small does not run out the way a tank does. It just delivers cooler water once the flow or the rise exceeds the rating.
Does a tankless water heater run out of hot water?
It does not store a tank of hot water, so it does not run out after a long shower the way a storage heater can. It can still fall behind. If the flow is higher than the rating at that temperature rise, the outlet temperature drops. That is why winter inlet temperature and simultaneous fixtures both belong in the sizing, and why two units are sometimes the answer.
Sources
- U.S. Department of Energy, Energy Saver, “Tankless or Demand-Type Water Heaters” and “Sizing a New Water Heater.” Demand units are sized from simultaneous flow and temperature rise. DOE’s sizing example uses a 50 °F inlet unless you know otherwise, and 120 °F for most uses.
- U.S. EPA WaterSense specifications for showerheads (2.0 gpm maximum) and lavatory faucets (1.5 gpm maximum).
- Flow rate calculator for a measured GPM. Fixture flow on this page is the demand, not a bucket test.
- 10 CFR 430.32, federal maximum flow rates. Showerheads 2.5 gpm at 80 psi. Kitchen and lavatory faucets 2.2 gpm at 60 psi.
- U.S. Consumer Product Safety Commission tap-water scald guidance, and DOE Energy Saver temperature guidance, both commonly cited for a 120 °F outlet.
- NFPA 70, National Electrical Code, 2023 edition, Article 422, appliances. Named here only. This page reports amps and does not select a breaker or a conductor. Gas pipe sizing is deferred.