Water Heater Sizing Calculator
Determine the right size tank or tankless water heater based on your household's hot water needs.
🔥 First Hour Rating Is a Measured Number
A tank is not sized by its volume. It is sized by its first hour rating — how much hot water it can actually deliver in one hour starting from a full, hot tank, which combines what is stored with what the burner or element can reheat while you draw. Two 50-gallon heaters can have quite different first hour ratings.
The rating is not a marketing figure. The US Department of Energy prescribes exactly how it is measured: for a tank of 20 gallons or more the test draws water at 3.0 ± 0.25 gallons per minute and ends each draw when the outlet temperature has fallen 15°F below the peak it reached, repeating for an hour. The Federal Trade Commission then requires the result on the yellow EnergyGuide label — storage models must show both rated volume and first hour rating, instantaneous models a maximum flow rate in GPM. DOE sorts storage heaters into four bands by that rating: very small under 18 gallons, low 18 to 50.9, medium 51 to 74.9, high 75 and over.
Formula Reference
This calculator assumes a first hour rating from a table of its own, keyed to occupant count and a low/medium/high usage setting — two people at "medium" give 55 gallons, four at "high" give 100. That table has no published basis; it is a planning shortcut, not an industry method. The recognised approach is a peak-hour demand worksheet: count the showers, baths, dishwasher and laundry cycles falling in your busiest hour, total the gallons, and pick a heater whose first hour rating equals or exceeds that total. DOE's Building America Solution Center bases its own tool on Air-Conditioning, Heating and Refrigeration Institute guidance and states the rule exactly that way.
The diversity factor is also an assumption — 0.35, 0.5 or 0.7 of connected fixture flow for low, medium and high usage, with no more than two of any one fixture counted. If in your house everything really does run at once, size on the sum instead.
🚿 Tankless Sizing Is a Flow Problem, Not a Volume One
An instantaneous heater has no reserve, so it is rated by how many gallons per minute it can raise by a given temperature. The energy follows from the weight of water: a gallon weighs about 8.34 pounds, an hour holds 60 minutes, and raising a pound one degree Fahrenheit takes one BTU — so BTU per hour is flow times 500 times the rise. Three gallons a minute at a 60°F rise is 90,000 BTU/hr; the same flow at an 80°F rise is 120,000. That is why inlet temperature decides the unit, not the number of bathrooms. A heater sold as "5 GPM" is 5 GPM at some stated rise, and at a colder inlet it delivers less. Measure your incoming water in winter rather than assuming it.
The fixture flows assumed here are 2.0 GPM for a shower, 4.0 for a bathtub fill, 1.5 for a kitchen sink or dishwasher, 2.0 for a clothes washer and 1.0 for a lavatory. For context, federal law caps showerheads at 2.5 GPM measured at 80 psi and faucets at 2.2 GPM at 60 psi, while an EPA WaterSense showerhead must not exceed 2.0 GPM — so the shower figure matches a WaterSense head and understates a pre-1994 one considerably.
❓ Questions People Actually Ask
How do I calculate what size water heater I need?
Work out your peak hour demand — the gallons of hot water your household draws in its busiest single hour, usually a weekday morning — then choose a heater whose first hour rating equals or exceeds it. That is the rule the Department of Energy's Building America Solution Center gives, based on Air-Conditioning, Heating and Refrigeration Institute guidance. First hour rating is printed on the EnergyGuide label because the Federal Trade Commission requires it there, alongside tank volume. For a tankless unit the equivalent question is flow, not gallons: work out the GPM you need running at once and the temperature rise from your measured winter inlet temperature, and read the manufacturer's rating at that rise rather than its headline figure.
Is a 40 gallon water heater enough for a family of 5?
Tank volume alone cannot answer that, which is why the label carries a first hour rating as well. A 40-gallon heater with a fast recovery can out-deliver a 50-gallon one with a slow burner over the hour that matters. Total the hot water your household actually draws in its busiest hour and compare that with the first hour rating on the label — the Department of Energy sorts storage heaters into bands by that rating, with 18 to 50.9 gallons counted as low usage and 51 to 74.9 as medium. If your peak hour lands above the rating you are looking at, the tank is too small regardless of its volume.
Is a 30 gallon water heater enough for 3 people?
Compare peak hour demand against first hour rating rather than head count against volume. A 30-gallon tank will normally fall in the Department of Energy's "low usage" band, meaning a first hour rating somewhere from 18 to 50.9 gallons, and the exact figure is on the EnergyGuide label of the specific model. Three people showering in the same hour can easily exceed the lower end of that band. Add up the draws in your busiest hour, then pick a rating at or above it. Sizing and fitting a water heater involves gas, venting, electrical supply and relief valve discharge, all of which are a licensed installer's job.
A planning estimate built on assumed occupancy and usage figures, not a peak-hour demand worksheet and not a manufacturer's selection. Confirm against the first hour rating printed on the EnergyGuide label of the model you are actually buying, and for a tankless unit against its rated output at your measured winter inlet temperature.
Nothing here addresses combustion air, venting, gas line sizing, electrical capacity, temperature and pressure relief valve discharge, or scald protection — all life-safety matters and all a licensed installer's responsibility.
The first hour rating test — the 3.0 ± 0.25 GPM draw rate, the 15°F cut-off and the four usage bands — is set out in the Department of Energy's test procedure for water heaters at 10 CFR Part 430, Subpart B, Appendix E. The requirement that storage heaters be labelled with both volume and first hour rating, and instantaneous heaters with a maximum flow rate, is at 16 CFR Part 305, the Federal Trade Commission's appliance labelling rule. The rule of choosing a rating at or above peak hour demand, and its basis in AHRI guidance, is from DOE and Pacific Northwest National Laboratory's Building America Solution Center. Maximum fixture flow rates are federal law at 10 CFR 430.32, and the 2.0 GPM figure is the EPA WaterSense showerhead specification. The 8.34 pounds per gallon behind the 500 factor follows from water density, which the USGS Water Science School gives as 62.408 lb/ft³ at 50°F. The questions above came from Google's People Also Ask panel for "water heater sizing calculator" on 31 July 2026.