Calculator
Plumbing Water Heater Sizing Calculator
Size storage tank or tankless water heaters from peak-hour demand — enter occupants, fixture usage in your busiest hour, groundwater temperature, and fuel type to get required First Hour Rating (FHR), GPM, and BTU/hr. Free, no signup.
Free
Customize usage
Morning rush preset for 4 occupants. 80 gal peak hour · 3× shower, 1× automatic dishwasher, 3× hand washing / face +2 more
Peak-hour worksheet
DOE fixture counts for your busiest hour — uses per hour, not daily totals.
Fixtures
Edit rows or add fixtures for your exact morning rush.
| Fixture | Uses | Gal/use | Total | Remove |
|---|---|---|---|---|
| 20 | 60 | |||
| 2 | 4 | |||
| 2 | 6 | |||
| 3 | 3 | |||
| 7 | 7 |
Peak hour total80 gal
Advanced options
4 occupantsNatural gasMidwest / centralΔT 70°F
Required first-hour rating
88 gal
Match on EnergyGuide label — not the tank size on the box
Sizing recommendations
Required FHR
88 galPeak 80 gal
Recommended tank
75 gal gas90 gal FHR · 45,000 BTU/hr input
Peak-hour breakdown
| Fixture | Uses | Gal |
|---|---|---|
| Shower | 3 | 60 gal |
| Automatic dishwasher | 1 | 7 gal |
| Hand washing / face | 3 | 6 gal |
| Shaving | 2 | 4 gal |
| Hand dishwashing / food prep | 1 | 3 gal |
Sizing notes
- Peak demand (80 gal) may exceed usable storage plus recovery — verify FHR on the EnergyGuide label, not tank size alone.
Industry benchmarks
- 1–2 people (FHR)
- 40–50 gal
- 3–4 people (FHR)
- 60–80 gal
- 4–5 people (FHR)
- 80–100 gal
- Tankless formula
- GPM × ΔT × 500
- Shower (DOE peak hour)
- 20 gal / use
- Hand wash (DOE/GAMA)
- 2 gal / use
- Electric tankless cap
- ~54,000 BTU/hr
What you'll get
Enter household or building profile
Set occupants, bathrooms, usage pattern, and application type. The calculator pre-fills the DOE peak-hour worksheet — or switch to manual fixture counts.
Set water temperatures
Pick your groundwater region for inlet temperature and confirm the 120°F setpoint. Temperature rise drives tankless BTU requirements.
Review tank and tankless sizing
Get required FHR for storage tanks, simultaneous GPM and BTU/hr for tankless, recommended nominal tank size, and warnings for electric feasibility or commercial diversity.
How it works
- 1
Enter household or building profile
Set occupants, bathrooms, usage pattern, and application type. The calculator pre-fills the DOE peak-hour worksheet — or switch to manual fixture counts.
- 2
Set water temperatures
Pick your groundwater region for inlet temperature and confirm the 120°F setpoint. Temperature rise drives tankless BTU requirements.
- 3
Review tank and tankless sizing
Get required FHR for storage tanks, simultaneous GPM and BTU/hr for tankless, recommended nominal tank size, and warnings for electric feasibility or commercial diversity.
Why plumbers size water heaters by peak-hour demand, not tank gallons
The number on the side of a water heater — 40 gallon, 50 gallon — is storage capacity, not delivery capacity. What matters for customer comfort is the First Hour Rating (FHR): how many gallons of hot water the unit delivers in the busiest hour starting from a full tank. A 40-gallon gas tank often delivers 65–70 gallons in that first hour because the burner reheats incoming water while you draw stored hot water. An electric 50-gallon tank may only deliver 58 gallons FHR despite larger storage.
Tankless units have no storage — they must heat all incoming water on demand. Industry sizing uses simultaneous fixture flow (GPM) multiplied by temperature rise and the constant 500: BTU/hr = GPM × °F rise × 500. This calculator implements the U.S. Department of Energy peak-hour worksheet for storage sizing and standard tankless BTU math, with groundwater regions, commercial diversity factors, and fuel-specific tank FHR lookup tables for field screening.
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View statesWater Heater Sizing Calculator FAQs
What is First Hour Rating (FHR)?
FHR is the gallons of hot water a storage water heater can supply in the first hour of use, starting with a full tank at setpoint. It combines stored volume plus recovery during that hour. Match FHR on the EnergyGuide label to your calculated peak-hour demand — not the tank size printed on the unit.
How do I size a tankless water heater?
List fixtures that run simultaneously, sum their flow rates in GPM, and multiply by temperature rise (setpoint minus inlet groundwater temp) and 500 to get required BTU/hr. Example: 6 GPM at 70°F rise = 210,000 BTU/hr. Whole-house electric tankless is often limited to ~54,000 BTU/hr on standard residential service.
What hot water usage values does this calculator use?
Peak-hour gallons follow the U.S. DOE Energy Saver sizing worksheet: 20 gal per shower, 7 gal per dishwasher cycle, 25 gal top-load washer, 15 gal front-load washer, and similar defaults. Override counts for your actual fixture layout and busiest hour.
When should I apply a diversity factor?
Residential single-family sizing uses 100% of peak-hour demand. Light commercial and multi-fixture buildings often apply 75–80% diversity per ASHRAE and manufacturer guides — not every fixture runs at peak simultaneously. Enable light commercial mode or reduce the diversity percentage manually.
