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Tankless flow-rate calculator: GPM, temperature rise and BTU/h

Count the fixtures that may run at the same time, pick the region that best describes your inlet water, and see the flow and temperature rise a tankless unit must deliver. Pure arithmetic from stated assumptions, not a sizing guarantee.

1. Fixtures that may run at the same time

Fixture flows are planning references for modern low-flow fixtures; older fixtures can draw more. Illustrative values, not a product claim.

2. Your inlet water

Winter is the case that matters. The hot-water setpoint is fixed at 105 °F.

Your result

Required flow
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Temperature rise
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Heat output
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Gas input at 82 % / 95 %
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Electric power
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Rated units stated at or above this result

The three best-rated gas units whose listing states at least the gas input needed by a non-condensing unit, and the three best-rated electric units stating at least the electric power needed. A stated maximum is not a guarantee at your rise.

Enter your fixtures above.

How to read the result

The calculator adds the flow of each fixture you selected, subtracts the assumed inlet temperature from a 105 °F setpoint to get the temperature rise, and multiplies flow × rise × 500 to give the heat output in BTU per hour (8.34 lb per gallon × 60 minutes). Dividing by 3,412 gives the same output in kW. For gas units the listing states an input, not an output, so the result also shows the input a unit of roughly 82 % (non-condensing) or 95 % (condensing) thermal efficiency would need; those bands are planning assumptions, not a measured figure for any listed unit.

Compare the required GPM at your rise with the maximum flow a listing states. Sellers usually print the maximum at a modest rise, so a unit that says 9.5 GPM may deliver far less at a 63 °F rise. The rise you need — not the brochure GPM — is what the installer sizes against.

Before choosing a heater

  1. Decide which fixtures genuinely run together on a winter morning.
  2. Find your winter inlet temperature from your utility or a local plumber.
  3. Read the stated maximum flow, fuel and input on the exact listing.
  4. Ask a licensed installer whether your gas line, venting route or electrical panel can serve that unit under local code.

This tool answers an arithmetic question and nothing else. It does not establish that a unit is safe or permitted in your house: gas piping, venting, combustion air, breakers and conductors are decided by a licensed plumber, gas fitter or electrician, and we give no installation guidance.

Read our research methodology and the sizing guide on GPM and temperature rise.

Frequently asked questions

How is the required flow calculated?

The calculator adds the flow of every fixture you expect to run at the same time, using planning values for modern low-flow fixtures (shower 2.0 GPM, bathroom sink 1.0, kitchen sink 1.5, dishwasher 1.5, clothes washer 2.0). Replace them with the flow of your own fixtures when you talk to an installer.

Why does the inlet temperature matter so much?

The heater has to raise the water from its inlet temperature to the outlet temperature. Colder inlet water means a larger rise, so the same unit delivers less flow. Winter is the case that matters, and well water or a municipal main can differ.

Does the calculator size my water heater?

No. It answers an arithmetic question and lists rated units whose listing states at least the resulting input or power. It does not measure or verify performance, and it is not an installation instruction: gas piping, venting and electrical service are decided by a licensed professional.