Trade Price Book
MenuClose

SZ-02Sizing · Plumbing

Water heater size calculator

Size a tank by its first-hour rating against the household’s busiest hour, or a tankless unit by flow × temperature rise. Then check a model’s label or spec sheet against the number.

Size the heater

Kind of water heater

Example: DOE’s worksheet — 3 showers, 1 shave and 1 hand dishwashing in the busiest hour. Change any number.

Look for a first-hour rating of at least Example

The busiest hour’s hot water. Compare it with “Capacity (first hour rating)” on the EnergyGuide label.

Look for a first-hour rating of at least 65 gallons, the peak-hour demand.

Change the numbers
Peak hour vs first-hour ratingDrawn to scale: every gallon the same height
Peak-hour demand against the first-hour rating, for the example, drawn to scalePeak-hour demand column, to scale: Shower 60 gallons, Shaving 2 gallons, Hand dishwashing or food prep 3 gallons; total 65 gallons. Look for a first-hour rating of at least 65 gallons.132Peak hourFHR to find65 gal≥ 65 gal
  1. Shower3 × 20 gal60 gal
  2. Shaving1 × 2 gal2 gal
  3. Hand dishwashing or food prep1 × 3 gal3 gal
  4. Peak hour65 gal

Gallons per use: DOE’s worksheet, which says its estimates “are based on averages from a variety of information published on websites.” DOE’s worksheet example prints 66 gallons; its items (3 showers, 1 shave, 1 hand dishwashing) add to 65.

Peak hour
65 gal3 kinds of use
Biggest use
60 galshower · 92%
Heater FHR
—from the label

3 × 20 + 1 × 2 + 1 × 3 = 65 gal

Look for a storage heater whose first-hour rating ("Capacity (first hour rating)" on the EnergyGuide label) at least matches this peak-hour demand.

Already showing the example.

What changes the size

  • One more shower in that hour adds 20 gal: look for at least 85 gal.
  • A measured 2.0 gpm shower for 8 minutes uses 16 gal, not DOE’s 20: time your own showers if you can.

Change the numbers

The busiest hour

Count each hot water use in the household’s busiest hour. Gallons per use are DOE’s; change them if you know better.

  • ShowerDOE: 20 gal per use
  • ShavingDOE: 2 gal per use
  • Hand dishwashing or food prepDOE: 3 gal per use
  • Automatic dishwasherDOE: 7 gal per use
  • Clothes washer, top-loaderDOE: 25 gal per use
  • Clothes washer, H-axis (front-loader)DOE: 15 gal per use
A heater to check

“Capacity (first hour rating)” at the top left of the EnergyGuide label. Heat pump models may not carry the label: use the maker’s literature.

First-hour rating to look for: 65 gal. Back to the result

How the math works

A tank and a tankless unit are rated in different terms, so each is sized against a different number from the house: a tank by the most hot water it can deliver in the first hour, a tankless unit by the flow it can heat through a given temperature rise.

  1. Peak-hour demandΣ gallons per use × uses in the busiest hourDOE’s worksheet method. Measured shower flow × minutes beats a flat 20 gallons when you have it.
  2. Tank: rating to look forfirst-hour rating ≥ peak-hour demandLook for a storage heater whose first-hour rating ("Capacity (first hour rating)" on the EnergyGuide label) at least matches this peak-hour demand.
  3. Tankless: flowΣ gpm of the fixtures running at onceDOE’s example: a 0.75 gpm faucet plus a 2.5 gpm shower head is 3.25 gpm.
  4. Riseoutlet °F − incoming °F
  5. Heat the water needs500 × gpm × rise = Btu/h500 = about 8.34 lb per gallon × 60 minutes. A rounded constant: 500.2 at 60 °F, 494.4 at 125 °F. Unconfirmed: No primary (DOE, AHRI, ASHRAE or manufacturer) statement of "500 × gpm × ΔT" was found; do not call it an industry standard.
  6. Input and kWBtu/h ÷ efficiency; kW = Btu/h ÷ 3,412Efficiency 1.0 for electric resistance is an assumption; use the spec sheet value when you have it. Current at 240 V is plain P = V × I. Breaker and wire sizes are not computed; the electrical code and the manufacturer decide them.
  7. Rated flow to look forgpm × rise ÷ 67A unit’s rated maximum flow is measured at a 67 °F rise. The same heat at your rise gives gpm × 67 ÷ rise; at a 70 °F rise that is about 4% less flow.
  8. Winter incoming water(mean + 6) − (0.4 + 0.01 × (mean − 44)) × spread ÷ 2NREL’s mains-water model on its coldest day: “mean” is the annual mean air temperature and “spread” the warmest minus the coldest monthly mean, both in °F. A model, not a measurement.
Where every number comes from 19 values, each with its source
Shower
20 gal per useU.S. DOE Energy Saver, "Sizing a New Water Heater" (archived, Internet Archive snapshot 2026-07-02), peak-hour worksheet. DOE: "Estimates are based on averages from a variety of information published on websites."
Shaving
2 gal per useU.S. DOE Energy Saver, "Sizing a New Water Heater" (archived, Internet Archive snapshot 2026-07-02), peak-hour worksheet. DOE: "Estimates are based on averages from a variety of information published on websites."
Hand dishwashing or food prep
3 gal per useU.S. DOE Energy Saver, "Sizing a New Water Heater" (archived, Internet Archive snapshot 2026-07-02), peak-hour worksheet. DOE: "Estimates are based on averages from a variety of information published on websites."
Automatic dishwasher
7 gal per useU.S. DOE Energy Saver, "Sizing a New Water Heater" (archived, Internet Archive snapshot 2026-07-02), peak-hour worksheet. DOE: "Estimates are based on averages from a variety of information published on websites."
Clothes washer, top-loader
25 gal per useU.S. DOE Energy Saver, "Sizing a New Water Heater" (archived, Internet Archive snapshot 2026-07-02), peak-hour worksheet. DOE: "Estimates are based on averages from a variety of information published on websites."
Clothes washer, H-axis (front-loader)
15 gal per useU.S. DOE Energy Saver, "Sizing a New Water Heater" (archived, Internet Archive snapshot 2026-07-02), peak-hour worksheet. DOE: "Estimates are based on averages from a variety of information published on websites."
Incoming water, default
50 °FU.S. DOE Energy Saver, "Sizing a New Water Heater" (archived, Internet Archive snapshot 2026-07-02): "Unless you know otherwise, assume that the incoming water temperature is 50ºF"
Outlet, default
120 °FU.S. DOE Energy Saver, "Sizing a New Water Heater" (archived, Internet Archive snapshot 2026-07-02): "For most uses, you'll want your water heated to 120ºF"
Scald caution
140 °FU.S. DOE Energy Saver, "Sizing a New Water Heater" (archived, Internet Archive snapshot 2026-07-02): 140 °F "increases the possibility of scalding"
Federal test rise
58 → 125 °F (67 °F)10 CFR 430 Subpart B, Appendix E §§2.3–2.5 (supply 58 °F ± 2); 10 CFR 430 Subpart B, Appendix E (nominal rise 67 °F)
Heat constant
500 Btu/h per gpm per °FSite arithmetic: ~8.34 lb/gal (USGS) × 60 min/h × 1 Btu per lb per °F (EIA Btu definition). A rounded constant: 500.2 at 60 °F, 494.4 at 125 °F. Unconfirmed: No primary (DOE, AHRI, ASHRAE or manufacturer) statement of "500 × gpm × ΔT" was found; do not call it an industry standard.
Btu per kWh
3,412U.S. EIA, Energy conversion calculators ("1 kilowatthour = 3,412 Btu"; NIST: 3,412.14)
Showerhead, federal maximum
2.5 gpm10 CFR 430.32 (eCFR, version 2026-10-01)(p): 2.5 gpm at 80 psi
Showerhead, WaterSense maximum
2 gpmU.S. EPA WaterSense, Showerheads
Bathroom faucet, federal maximum
2.2 gpm10 CFR 430.32 (eCFR, version 2026-10-01)(o): 2.2 gpm at 60 psi
Bathroom faucet, WaterSense maximum
1.5 gpmU.S. EPA WaterSense, Bathroom Faucets
Kitchen faucet, federal maximum
2.2 gpm10 CFR 430.32 (eCFR, version 2026-10-01)(o): 2.2 gpm at 60 psi
Mains-water model
NREL Eq. 21NREL, 2014 Building America House Simulation Protocols (NREL/TP-5500-60988), Eq. 21; applied to NOAA NCEI 1991–2020 normals (site arithmetic)
Electric efficiency
100%, if you enter itSite assumption for electric resistance heating (label it "assumption")

Before you use the number

  • Match the first-hour rating, not the tank size. The rating depends on the tank, the heat source and the burner or element. Heat pump water heaters may not carry the EnergyGuide label: take the rating from the maker’s literature.
  • DOE’s gallons are averages “based on averages from a variety of information published on websites,” and the worksheet does not estimate daily use. DOE’s worksheet example prints 66 gallons; its items (3 showers, 1 shave, 1 hand dishwashing) add to 65.
  • Tankless capacity depends on the rise. A rated maximum flow comes from a 67 °F rise (58 °F in, 125 °F out). In colder water the same unit delivers less; check the maker’s flow-and-rise data at your rise.
  • Incoming water changes with the season and the place. A measured winter cold-tap temperature beats the model, and the model beats the 50 °F default. Winter inlet may be colder than 50 °F.
  • Scalding. DOE cautions that 140 °F increases the possibility of scalding. IPC §412.3 (IPC 2018 text; check the edition your jurisdiction enforces) requires shower valves that limit the maximum setting to 120 °F. The outlet setting here is not a fixture temperature.
  • Federal fixture maximums are ceilings. Real flows are often lower; older fixtures can be higher.
  • Not a gas, venting, combustion-air or electrical calculation. Those come from the code your jurisdiction has adopted — the IPC or UPC for plumbing, in the edition and with the amendments your state uses — and from the maker’s installation instructions.

Sources

  1. Sizing a New Water Heater (archived copy, July 2, 2026; the live page returns 404)U.S. Department of Energy, Energy Saver (via Internet Archive)
  2. Appendix E to Subpart B of Part 430: Uniform Test Method for Measuring the Energy Consumption of Water HeaterseCFR (Office of the Federal Register)
  3. 10 CFR 430.32: Energy and water conservation standards (showerheads, faucets)eCFR (Office of the Federal Register)
  4. ShowerheadsU.S. EPA WaterSense
  5. Bathroom FaucetsU.S. EPA WaterSense
  6. British thermal units (Btu)U.S. Energy Information Administration
  7. Energy conversion calculators (1 kWh = 3,412 Btu)U.S. Energy Information Administration
  8. Water DensityU.S. Geological Survey
  9. 2014 Building America House Simulation Protocols (NREL/TP-5500-60988)National Renewable Energy Laboratory (via OSTI)
  10. U.S. Climate Normals (1991–2020)NOAA National Centers for Environmental Information
  11. DHW First Hour Rating (RED Calc)PNNL Building America Solution Center
  12. 2024 International Plumbing Code (free to read)International Code Council, Digital Codes
  13. Read IAPMO Codes Online (Uniform Plumbing Code)IAPMO

  • 3.02Guide

    How to Price a Water Heater Replacement From Your Numbers

    A contractor-side worked example: heater, parts, permit, haul-away and labor at your shop rate, a margin check, and why average cost pages miss your job.

    $1,437.50 ÷ $1,085 − 1 = 32.49%

    Length
    7 min
    Updated

  • 3.01Guide

    Gallons in a Pipe by Size: Copper, PEX, PVC and CPVC

    Gallons per 100 feet for copper K, L and M, PEX, PVC Schedule 40 and CPVC, computed from published inside diameters, with wait-time and flushing math.

    π ÷ 4 × ID² × 12 ÷ 231 = 0.0408 × ID²

    Length
    7 min
    Updated

Tools for the job

Tool listHVAC + Plumbing3 itemsSpecs read

As an Amazon Associate, Trade Price Book earns from qualifying purchases. Amazon links are marked “(paid link)”; prices aren’t shown here.

  1. Airflow and static pressureFieldpiece SDMN6

    SDMN6 Dual Port Manometer and Pressure Switch Tester

    • Dual-port manometer reading to ±60 inWC at 0.01 inWC resolution (±0.02 inWC up to 2 inWC).

    Specs: Fieldpiece product page, read

    View on Amazon for Fieldpiece SDMN6(paid link)

  2. Combustion and electrical testingtesto 0564 3002 82

    Next-Gen testo 300 Smoke Edition Combustion Analyzer Kit

    • Kit: testo 300 with O2 sensor and CO sensor (0 to 4,000 ppm), mains unit, basic flue-gas probe, spare filters, EasyHeat software, case and Bluetooth connector.

    Specs: testo product page, read

    View on Amazon for testo 0564 3002 82(paid link)

  3. Testing and water heater serviceMilwaukee 2771-20

    M18 Transfer Pump

    • Self-priming pump that moves up to 480 gallons per hour, with up to 18 ft of lift and 75 ft of head height.

    Specs: Milwaukee product page, read

    View on Amazon for Milwaukee 2771-20(paid link)

How tools are chosen for gear listsAll 20 HVAC tools