Free · Tank & tankless · Shows the math
Size a tank by the First Hour Rating your busiest hour needs, or a tankless unit by the flow it must deliver at your temperature rise.
Minimum First Hour Rating 36 gallons per hour
36gal/hr
Choose a tank whose First Hour Rating is at least 36 gallons. It is printed on the yellow EnergyGuide label and the spec sheet. Tank capacity alone is not the number to compare.
Hot fraction — Standard Shower
(Use temp − Inlet) ÷ (Setpoint − Inlet)
(105 − 50) ÷ (120 − 50) equals 0.786
Peak-hour hot water — Standard Shower
Uses × Flow × Duration × Hot fraction
2 × 2.5 gpm × 8 min × 0.786 equals 31.4 gal
Hot fraction — Kitchen Faucet
(Use temp − Inlet) ÷ (Setpoint − Inlet)
(110 − 50) ÷ (120 − 50) equals 0.857
Peak-hour hot water — Kitchen Faucet
Uses × Flow × Duration × Hot fraction
1 × 2.2 gpm × 2 min × 0.857 equals 3.8 gal
Peak-hour demand
Sum of peak-hour hot water
31.4 gal + 3.8 gal equals 35.2 gal
Design margin
Peak-hour demand × Margin
35.2 gal × 0% equals 0.0 gal
Minimum First Hour Rating
Peak-hour demand + Design margin, rounded up to a whole gallon
35.2 gal + 0.0 gal equals 36 gal/hr
Same formulas, defaults and wording as the iPhone app. Read the methodology and sources.
A tank and a tankless unit fail in different ways, so they are sized by different ratings. Mixing the two up is where most sizing goes wrong.
Tank · storage
A tank runs out when the busiest hour uses more hot water than it can store and reheat. So you add up the hot water drawn in that hour and choose a tank whose First Hour Rating is at least that big.
peak-hour demand = Σ uses × gallons per use × hot fraction minimum FHR = peak-hour demand, rounded up
One 8-minute shower at 2.5 gpm draws 15.7 gal of hot water from a 120°F tank with 50°F incoming water.
Tankless · on demand
A tankless unit runs out of heat, not water. It fails when too many fixtures open at once for the temperature rise it has to deliver. So you add up the hot-water flow that can happen at the same moment.
required flow = Σ fixtures × gpm × hot fraction heat load = 500 × gpm × rise (BTU/hr)
Two showers need 3.9 gpm at a 70°F rise, about 137,500 BTU/hr into the water.
A 105°F shower from a 120°F heater with 50°F incoming water is 79% hot water, not 100%. Counting the full flow overstates that shower’s tankless load by 27%. The calculator uses the blend ratio for each fixture. How much hot water a shower really uses.
Advertised maximum GPM is measured at a 67°F rise (58°F in, 125°F out). With 40°F winter water and a 120°F setting, the same two showers need 4.1 gpm at an 80°F rise, and a unit delivers less flow at that rise than its headline number. Convert a rated GPM to your rise.
“A family of four needs 50 gallons” ignores recovery. A gas tank that reheats quickly can deliver more in an hour than a larger electric tank. Compare First Hour Rating, the figure on the EnergyGuide label. 40 vs 50 gallon, by the numbers.
Water Heater Sizing Calculator for iPhone
The app runs the same sizing engine as this site, offline. Basic tank and tankless sizing is free. Lifetime Pro, a one-time $4.99 purchase, adds tank vs. tankless comparison, custom fixtures, saved jobs with notes and PDF reports you can send to a customer.
Choose Tank or Tankless. Enter your incoming water temperature and the heater temperature, then add the fixtures you use. For a tank, count uses during your single busiest hour; the result is the minimum First Hour Rating to look for. For tankless, count what could run at the same moment; the result is the flow in gallons per minute the unit must deliver at your temperature rise.
Tank capacity only tells you how much water is stored. First Hour Rating is the hot water a fully heated tank can deliver in one hour, including what it reheats during that hour, measured in the federal test in 10 CFR 430 Appendix E. Two 50-gallon tanks can have very different First Hour Ratings, so FHR is the number to compare with your peak-hour demand.
Use the coldest water you expect, ideally measured at a cold tap in winter. The calculator starts at 50°F, which is on the cold side for much of the U.S. Colder incoming water means a bigger temperature rise and more heat needed. The incoming water temperature guide has city estimates if you can't measure.
A shower at 105°F is a blend of hot and cold water. With 50°F incoming water and a 120°F heater, only about 79% of the shower's flow passes through the heater. The calculator counts that hot share, the same way manufacturers' sizing guides do, and shows the total fixture flow for reference.
Yes. The web calculator is a line-by-line port of the app's sizing engine and is tested against it on hundreds of cases, including rounding and the wording of every warning. The app adds saved jobs, tank vs. tankless comparison, custom fixtures and PDF reports.
It is designed for homes and light residential use. Commercial water heating is usually sized with occupancy-based methods from ASHRAE or manufacturer tools, which this calculator does not replicate.
