What Size Water Heater Do I Need?
This free water heater size calculator uses the method the Department of Energy actually recommends — and it is not "50 gallons for a family of four." Tank size is the wrong number to shop by: a 40-gallon gas heater with a strong burner can deliver more hot water in an hour than a 50-gallon electric, because the burner keeps recovering while you draw. The number that captures this is the First Hour Rating (FHR), printed on every yellow EnergyGuide label, measured under the same federal test for every brand.
The method is a one-hour inventory. Count what happens in your busiest hot-water hour — usually the morning rush — using the official AHRI/DOE worksheet values: 20 gallons per shower, 14 per dishwasher load, 32 per clothes-washer load, and so on. The total is your Peak Hour Demand, and DOE guidance is to buy a heater whose FHR meets it within a gallon or two. This calculator runs that worksheet interactively and reproduces its published example exactly.
The results also tell you what the number means at the store: which FHR bands typical 40- and 50-gallon gas and electric units actually post (they vary widely within a nominal size — that's the point), why electric tanks run 20–30% larger for the same FHR, and the cheapest capacity upgrade there is — moving a laundry load off the peak hour. Considering tankless instead? That's a different method (flow rate and temperature rise, not FHR) — and a different calculator on this site. Free, no signup.
Water Heater Size Calculator
What size water heater do you need? Count your busiest hour's hot-water uses — the answer is a First Hour Rating from the EnergyGuide label, not a tank size, and that distinction is why "a 50-gallon" is the wrong way to shop. Free, no signup.
Your busiest hot-water hour
Back-to-back showers count individually. If laundry or the dishwasher can wait an hour, leave them out — shifting them off the peak is free FHR.
Calculation Formulas
The AHRI/DOE worksheet assigns hot-water gallons to each activity, explicitly assuming no conservation measures — a deliberately safe basis.
Example:
3 showers (60) + shave (2) + shampoo (4) + hand dishes (4) = 70 gallons — the worksheet's own published example.
DOE guidance sizes by First Hour Rating — the yellow EnergyGuide label number — not by tank gallons or household headcount.
Example:
PHD 70 → shop for FHR ≥ 70.
Measured under the federal UEF test: fully heated tank, repeated 3 GPM draws until a 25°F outlet drop, recover, repeat for one clock hour.
Example:
A 40-gal gas unit with a strong burner can post FHR 90+; a 40-gal electric posts ~55.
Gas recovers roughly 40–50 GPH on a 50-gallon vs ~21 GPH electric — the burner does real work during the draw hour.
Example:
FHR-70 duty: a 40–50-gal gas unit, or a 50–65-gal electric.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Shower / bath | 20 gal each | AHRI/DOE worksheet values (no conservation assumptions). |
| Dishwasher / clothes washer | 14 / 32 gal per load | The washer is the biggest single line — shifting laundry off the peak hour is free capacity. |
| Small uses | shave 2 · hands/face 4 · shampoo 4 · hand dishes 4 · food prep 5 | Per the same worksheet. |
| FHR test | 125°F outlet, 3 GPM draws, 1 clock hour | 10 CFR 430 Subpart B, Appendix E (UEF method). |
| Worked example | PHD 70 → FHR ≥ 70 | The verbatim AHRI worksheet anchor this calculator reproduces. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
AHRI / DOE sizing worksheet(AHRI "Worksheet for Estimating Peak Hour Demand/First Hour Rating")
View StandardThe official per-activity gallon values and peak-hour method this calculator implements.
Key Requirements:
- •Sum busiest-hour activity gallons
- •Match FHR to the total
ENERGY STAR / EnergyGuide(FHR at 125°F per UEF method)
View StandardFHR is a regulated, label-printed figure — comparable across brands because the test is federal.
Key Requirements:
- •Read FHR off the yellow label
Federal test procedure(10 CFR 430 Subpart B, Appendix E)
View StandardThe Uniform Energy Factor test that defines FHR and draw patterns.
Key Requirements:
- •3 GPM draws, 25°F cutoff, one clock hour
IRC(IRC P2801.1)
View StandardThe code itself requires only "adequate capacity" — the 30/40/50-gallon-by-headcount folklore is convention, not code.
Key Requirements:
- •Adequate capacity (performance-based)
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Permits on replacement
Almost universal
Water heater replacement requires a permit nearly everywhere — California requires one for every install including like-for-like swaps, with no emergency exemption. Some large cities restrict installs to licensed plumbers.
Regional Examples:
Fuel availability
The gas/electric fork
Homes without gas service face the electric FHR penalty — or the heat-pump water heater alternative, which posts strong FHRs at high efficiency but needs space, condensate, and tolerable noise.
Regional Examples:
Your peak hour is yours
The worksheet is a default
A household that showers at night and runs laundry midday has a far lower peak than the same headcount doing everything at 7am. The activity counts — not averages — are the input.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
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How to Use This Calculator
- Think of your single busiest hot-water hour — for most households, the weekday morning rush.
- Count each activity in that hour with the +/− steppers: showers, baths, shaving, dishwasher and clothes-washer loads. Back-to-back showers each count.
- Pick your fuel — gas and electric post very different FHRs at the same tank size, and the guidance adjusts.
- Click Calculate to get your Peak Hour Demand and the First Hour Rating to shop for.
- At the store (or on the spec sheet), read the FHR off the yellow EnergyGuide label and buy a unit at or above your number — ignore the nominal gallons.
The free capacity upgrade
The clothes washer is the single biggest line on the worksheet at 32 gallons per load. If laundry can wait until mid-morning, it leaves your peak hour entirely — often the difference between needing a 65-gallon electric tank and a standard 50. Schedule-shifting is free; bigger tanks are not.
Frequently Asked Questions
What size water heater do I need for a family of 4?
Count the busiest hour, not the people. Four people who shower back-to-back in the morning (4 × 20 = 80 gallons) need an FHR around 80 — that's 50-gallon-gas territory or a 65–80-gallon electric. The same four people showering across morning and evening might peak at 40–50 gallons and do fine with much less. The AHRI/DOE worksheet method this calculator runs beats every headcount chart, because the headcount chart can't see your schedule.
What is First Hour Rating (FHR) on a water heater?
The maximum gallons of hot water a fully heated storage heater can deliver in one hour, measured at a 125°F outlet under the federal UEF test (10 CFR 430 Appendix E) — repeated 3 GPM draws until the outlet drops 25°F, recover, repeat for one clock hour. It's printed on the yellow EnergyGuide label of every storage heater sold in the US, making it the one directly comparable number across brands. FHR = usable stored hot water + one hour of burner or element recovery.
Is a 40 or 50 gallon water heater better?
Wrong question — compare FHRs, not tanks. Rheem's 40-gallon gas models post FHRs anywhere from 51 to 93 depending on burner input; a 40-gallon electric posts around 55. A high-input 40-gallon gas unit outdelivers a modest 50-gallon one. Compute your Peak Hour Demand, then buy whichever unit — 40 or 50 gallons — has a label FHR at or above it. The nominal tank size is a storage number; FHR is the performance number.
Why do gas and electric water heaters with the same tank size perform differently?
Recovery rate. A 50-gallon gas heater reheats roughly 40–50 gallons per hour; a standard 50-gallon electric manages about 21. During your peak hour the gas burner is effectively adding a second, invisible tank. That's why electric units need 20–30% more nominal capacity for the same FHR — and why heat-pump water heaters, which post strong FHRs at far better efficiency, are worth pricing for all-electric homes.
Do I need a permit to replace a water heater?
Almost everywhere, yes — even like-for-like swaps. California requires a permit for every water heater installation with no emergency exception, and some large cities restrict installs to licensed plumbers. Inspectors check the temperature-and-pressure relief discharge, seismic strapping where required, combustion air, and venting — the failure points that make unpermitted swaps genuinely risky. Budget the permit; it's a small fraction of the job.
Should I get a tankless water heater instead?
Different sizing math entirely: tankless units are sized by simultaneous flow (GPM) and temperature rise, not FHR — a whole-home unit must heat the water as fast as you use it, forever, rather than storing it up. Tankless wins on endless hot water and standby losses; tank wins on install cost and simplicity, and electric tankless in particular demands enormous electrical capacity (54–150 amps). If you're weighing electric tankless, check our tankless breaker calculator for what your panel would need to supply.