PlumbMath

Tankless water heater sizing calculator

From the fixtures that may run at the same time, the incoming water temperature and the set point, this tool gives the hot water flow, the temperature rise and the heat input the heater needs at the efficiency you enter. The result is a number to compare with the manufacturer's flow and temperature rise data. It does not pick a model. It runs in your browser. Nothing you enter is sent anywhere.

1. Fixtures running at the same time

Add each fixture that may draw hot water at the same moment, with how many of them and the flow of each. The presets are the maximum flow a conforming showerhead or faucet can pass (10 CFR 430.32, or 2024 IPC Table 604.4) or the design flow the distribution system must deliver at the outlet (2024 IPC Table 604.3), each labelled with its source. Overwrite a preset with the rate marked on the fixture where you have it. The full fixture flow is counted as hot water, which never understates the heat.

Counts from 1 to 20 of each, flows from 0.1 to 15 gpm, total up to 30 gpm. Those limits are typing error guards of this tool; above 30 gpm the job is commercial sizing, which this tool does not cover.

2. Temperatures

Incoming and set point
Measure the cold water at the job, in winter when sizing matters most. Hints are below. From 33 to 90 °F.
120 °F is prefilled because 2024 IPC 412.3 and 412.5 limit shower, tub shower and tub filler water to 120 °F at the point of use. From 100 to 150 °F.
Hints for the incoming water temperature

No current federal or state table of incoming water temperature was found when this tool was built. The one public source opened is a USGS paper from 1925. Use it as a rough guide only and measure at the job.

  • Ground water from wells 20 to 200 ft deep is about 3 to 6 °F above the mean annual air temperature of the place (WSP 520-F, pages 97 to 98).
  • Plate VIII of the same paper, a contour map of well water temperature at 30 to 60 ft, reads about:
    • 37 °F along the northern border (Montana, North Dakota, Minnesota, northern Maine)
    • 42 °F across the northern tier and northern New England
    • 47 °F through Minneapolis, Milwaukee, Detroit and Boston
    • 52 °F through Omaha, Chicago, Cleveland, New York and Salt Lake City
    • 57 °F through Kansas City, St. Louis, Washington and San Francisco
    • 62 °F through Oklahoma, Tennessee, North Carolina, Virginia and Los Angeles
    • 67 °F through Dallas, Birmingham, Atlanta, Charleston and southern Arizona
    • 72 °F along the Gulf coast, north Florida and the lower Colorado River
    • 77 °F in south Florida
  • The map is from 1925, drawn from the air temperature normals of that time, and is generalized. Surface water supplies vary by season; the paper gives 32 to 34 °F as the minimum in the coldest months. Measure the cold water at the job in winter, when sizing matters most.
  • The federal water heater test (10 CFR 430 Appendix E, section 2.3) uses 58 °F supply water with a 125 °F outlet, a 67 °F rise. That is a laboratory test condition, not a figure for any job, so it is not prefilled.

Source: USGS Water-Supply Paper 520-F, W. D. Collins, 'Temperature of water available for industrial use in the United States' (1925), pages 97 to 98 and Plate VIII.

3. Heater

Efficiency and fuel
From the manufacturer's specification sheet: the steady state share of the fuel input that goes into the water, for example 0.95 for 95 percent. If the sheet prints only the Uniform Energy Factor (UEF), using it is an approximation; the UEF comes from a 24 hour test that includes standby and cycling losses, so it slightly overstates the input. No default is offered. From 0.50 to 1.00.
Fuel

Result

What this tool does not do

  • Choose a model or brand. Read the unit's flow at your temperature rise from the manufacturer's chart.
  • Gas pipe sizing (see the gas pipe sizing calculator), venting, combustion air, condensate, electrical service and wire size.
  • Recirculation, whole house demand by number of occupants, and water pressure or pressure drop at the fixtures.
  • Storage, hybrid and solar water heaters, and commercial or multi unit jobs above 30 gpm.
  • A mix temperature at the fixture. The full fixture flow is counted at the set point rise.

The equation, the constants with their derivations, the sources of every preset flow and the open questions are on the methodology page. Sources were last checked on October 4, 2026.