How to size a tankless heater
A tank stores hot water, but a tankless unit heats it as it flows, so it is sized by flow and not by gallons. Two things set the size. One is how much water runs at the same time. The other is how far it has to be heated. Count what truly overlaps on your busiest morning: a shower, a second shower, a faucet in the kitchen.
Then find your incoming water temperature. Run the cold tap for a few minutes in midwinter and hold a kitchen thermometer in it. DOE says to assume 50 degrees unless you know better, and a cold northern winter can run lower. Take that from your target, usually 120 degrees, and you have the temperature rise, the number every maker's chart is built on.
Flow figures are on the fixtures, if you can read them. EPA WaterSense gives 2.5 gallons a minute for a standard showerhead and 2.2 for a standard bathroom faucet, and WaterSense models use 2.0 and 1.5 or less. A low flow head can drop you a whole size of unit.
A worked example
DOE's guide prints this one, as far as the flow and the rise: a 2.5 gallon a minute shower and a 0.75 gallon a minute faucet, with water coming in at 50 degrees and going out at 120. These lines come from the same formula the calculator above runs.
A shower and a faucet at the same time
- Flow: 2.5 + 0.75 = 3.25 gpm
- Rise: 120 − 50 = 70 degrees
- Gas output: 3.25 × 500 × 70 = 113,750 BTU/h
- Or electric: 3.25 × 70 ÷ 6.83 = 33.3 kW
Now drop the incoming water to 35 degrees for a hard winter, and the same flow needs 138,125 BTU per hour. Note that the gas figure is output. If your model's label gives an efficiency, enter it, and the input figure goes up. If the kilowatts look alarming, they are: a large electric unit may need more service than your house has. For the tank alternative, see the water heater size calculator.
Tankless size for common situations
These assume standard flows of 2.5 for a shower and 2.2 for a faucet, with a 70 degree rise. With water-saving fixtures the numbers come down, so use yours if you know them.
| At the same time | Flow | Gas output, rise of 70 | Electric, rise of 70 |
|---|---|---|---|
| One faucet | 2.2 gpm | 77,000 BTU/h | 22.5 kW |
| One shower | 2.5 gpm | 87,500 BTU/h | 25.6 kW |
| A shower and a faucet | 4.7 gpm | 164,500 BTU/h | 48.2 kW |
| Two showers | 5.0 gpm | 175,000 BTU/h | 51.2 kW |
| Two showers and a faucet | 7.2 gpm | 252,000 BTU/h | 73.8 kW |
| Three showers | 7.5 gpm | 262,500 BTU/h | 76.9 kW |
Mistakes that leave you with cold water
- Sizing in summer. Your water may be many degrees colder in January, and that is when a small unit struggles.
- A flow with no rise. A unit rated at 9 gpm is only that at some particular rise. Read its chart at yours.
- Adding every fixture. Count what runs at once. Everything in the house at the same time is not your real load.
- Forgetting the upgrades. Gas pipe, venting and electrical service are often where the budget goes, as MEP Academy notes.
- A conversion from tank size. No source gives a way to turn gallons into flow, and the idea does not hold up.
Questions
What size tankless water heater do I need?
One that can deliver the flow of everything running at once, at your temperature rise. DOE's example of a 2.5 gpm shower and a 0.75 gpm faucet needs 3.25 gpm at a 70 degree rise. Check the model's flow chart at your own rise.
How does groundwater temperature affect a tankless heater?
Colder water cuts the flow directly. A unit that gives 5 gpm at a 70 degree rise gives about 3.9 at a 90 degree rise. DOE says cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet. Size for winter, not summer.
What size tankless water heater for a family of 4?
About 3 to 6 gallons a minute, depending on how many showers overlap. Two showers at 2.5 gpm are 5 gpm. At a 70 degree rise, that takes about 175,000 BTU per hour of gas output, or 51 kW of electric.
What size tankless water heater to replace a 50 gallon?
There is no gallons to gpm conversion, because the two are sized differently. A tank stores hot water and a tankless unit heats on demand. Count what runs at once, enter that here, and compare with the model's flow chart.
Will electric work, or do I need gas?
DOE says a typical electric unit gives about 2 gpm and a gas one 5 gpm at a 70 degree rise. A big electric unit may also need much more electrical service than your house has. Ask an electrician before you buy one.
Does a tankless heater save energy?
It avoids the heat a tank loses while it sits, but how much that saves depends on the model and your use. Compare the labels of the models you are looking at. The cost of the upgrades it may need can matter too.
This tankless size is a rule of thumb, and models perform differently at the same rating, so read the manufacturer's flow chart at your own temperature rise. Winter inlet water is the hard case, and cooler water cuts the flow. Gas piping, venting and electrical service often need upgrades, so have a licensed installer check them.
Sources and review
- Sizing a new water heater, 2017 archive, U.S. Department of Energy, 2017.
- Showerheads, EPA WaterSense, 2026.
- Bathroom faucets, EPA WaterSense, 2026.
- How to size tankless water heaters, MEP Academy, no date shown, read November 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Gas output
BTU per hour = Flow × 500 × Rise
- Flow
- Gallons a minute through the unit
- Rise
- Target temperature less incoming temperature
Worked through: 4.5 × 500 × 70 = 157500 BTU/h
Electric
Kilowatts = Flow × Rise ÷ 6.83
- 6.83
- Derived from 3,412 BTU per kWh and 499.8
Worked through: 2 × 70 ÷ 6.83 = 20.5 kW
The constants, and where they come from
STANDARD_SHOWER_GPM
Published figureFlow of a standard showerhead, in gallons per minute
Applies to: Standard fixtures. WaterSense heads flow 2.0 or less
Source: WaterSense: Showerheads, U.S. Environmental Protection Agency, published 2026, read 2026-09.
EPA WaterSense, and DOE's example uses the same figure. You can enter your own.
STANDARD_FAUCET_GPM
Published figureFlow of a standard bathroom faucet, in gallons per minute
Applies to: Standard fixtures. WaterSense faucets flow 1.5 or less
Source: Bathroom faucets, EPA WaterSense, published 2026-06, read 2026-11.
EPA WaterSense. We could not read EPA's page for kitchen faucets, so enter your own for those.
DEFAULT_INLET_F
Published range, narrowedIncoming water temperature assumed if you do not know yours, in degrees F
Applies to: A starting point. Size for winter, when water is coldest
Source: Sizing a new water heater, 2017 archive, U.S. Department of Energy, published 2017-01, read 2026-11. This source is about 9 years old, and it is the one the figure rests on.
From a January 2017 archive, because the live DOE page returned 404. No primary regional map was read, so measure your own if you can.
DEFAULT_TARGET_F
Published range, narrowedTarget hot water temperature, in degrees F
Applies to: Most uses. DOE says 140 for a dishwasher without its own heater
Source: Sizing a new water heater, 2017 archive, U.S. Department of Energy, published 2017-01, read 2026-11. This source is about 9 years old, and it is the one the figure rests on.
From a January 2017 archive, because the live DOE page returned 404.
TYPICAL_RISE
Published range, narrowedThe temperature rise DOE states typical flows at, in degrees F
Applies to: DOE's general statement. Models vary
Source: Sizing a new water heater, 2017 archive, U.S. Department of Energy, published 2017-01, read 2026-11. This source is about 9 years old, and it is the one the figure rests on.
From a January 2017 archive, because the live DOE page returned 404.
TYPICAL_GAS_GPM
Published range, narrowedTypical flow through a gas unit at that rise, in gallons per minute
Applies to: A typical unit. Read your model's chart
Source: Sizing a new water heater, 2017 archive, U.S. Department of Energy, published 2017-01, read 2026-11. This source is about 9 years old, and it is the one the figure rests on.
From a January 2017 archive, because the live DOE page returned 404.
TYPICAL_ELECTRIC_GPM
Published range, narrowedTypical flow through an electric unit at that rise, in gallons per minute
Applies to: A typical unit. Read your model's chart
Source: Sizing a new water heater, 2017 archive, U.S. Department of Energy, published 2017-01, read 2026-11. This source is about 9 years old, and it is the one the figure rests on.
From a January 2017 archive, because the live DOE page returned 404.
BTU_PER_GPM_DEGREE
DefinitionalBTU per hour to heat one gallon a minute by one degree
Applies to: Water at ordinary temperatures
Source: How to size tankless water heaters, MEP Academy, no date on the page, read 2026-11.
8.33 pounds a gallon × 60 minutes × 1 BTU per pound per degree is 499.8, rounded to 500.
GPM_DEGREES_PER_KW
DefinitionalGallons a minute times degrees handled by one kilowatt
Applies to: Water at ordinary temperatures
True by definition, so there is nothing to cite. The derivation is below.
Our own derivation: 3,412 BTU per kWh ÷ 499.8 is 6.827. We did not find it printed by a maker.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Published range, narrowed:
- a source publishes a range and this calculator uses a representative value from it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the tankless water heater size calculator
- 1
Think of what runs at the same time on your busiest morning
- 2
Enter your coldest incoming water temperature if you can measure it
- 3
Choose your target temperature
- 4
Read the flow, the rise and the power needed
- 5
Compare that against the model's flow chart at this rise
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- It sizes for what runs at the same time, not for every fixture in the house
- Incoming water starts at 50 degrees, DOE's assumption. Measure yours, because winter water in the north can be colder
- BTU output does not include efficiency, unless you enter a figure from the model's label
- Fixture flows are standard ones. Water-saving fixtures use less, so enter your own
Pro tips
- Measure your cold water in midwinter with a kitchen thermometer
- Fit water-saving heads, since each lowers the flow you have to heat
- Ask the installer to confirm gas pipe size, not just the unit's rating