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 usageMorning 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.
FixturesEdit 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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Water Heater Sizing Calculator FAQs
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.
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.
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.
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.
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