Water Heater Calculator
Size water heater capacity and peak demand
Number of people in home
Number of bathrooms in home
First Hour Rating
—
gal/hr
Peak Demand
—
gal/hr
Gallon Capacity
—
gal
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
An undersized water heater leaves you with cold showers; an oversized one wastes energy. Our calculator balances capacity, first hour rating, and peak demand.
- Enter the number of people in the household
- Enter the number of bathrooms
- Review gallon capacity, FHR, and peak demand
- Match FHR to peak usage for comfort
Expert Insights
Smart analysis for your Plumbing project
Peak Demand Sizing
Morning rush is the highest demand period. AI estimates 3 gallons per person plus 5 for appliances to size for the busiest hour.
First Hour Rating Match
FHR should meet or exceed peak demand. AI recommends 75% of tank capacity as FHR for quick recovery gas and electric heaters.
Energy Efficiency Tips
Heat pump and tankless heaters cut energy use 25-50%. AI flags when a high-efficiency model pays back faster than standard tanks.
Calculation Formula
Gallon Capacity = Household Size x 12 + Bathrooms x 5 + 10
First Hour Rating = ceil(Capacity x 0.75)
Peak Demand = Household Size x 3 + 5
Water Heater Type Comparison
| Type | Lifespan | Efficiency | Typical Size | Best For |
|---|---|---|---|---|
| Conventional Tank (Gas) | 10-15 years | 60-70% | 40-80 gal | Large families |
| Conventional Tank (Electric) | 10-15 years | 90-95% | 40-80 gal | Small households |
| Tankless (Gas) | 20+ years | 80-95% | On-demand | Space saving |
| Heat Pump | 10-15 years | 200-300% | 50-80 gal | Warm climates |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- U.S. Department of Energy (DOE) — Water Heater Sizing & Energy Efficiency Guidelines. Learn more
- American Society of Plumbing Engineers (ASPE) — Domestic Water Heating Design Handbook. Learn more
- International Code Council (ICC) — International Plumbing Code (IPC) — Water Heater Requirements. Learn more
Frequently Asked Questions
What size water heater do I need for a family of 4?
A family of 4 with 2 bathrooms typically needs a 50-60 gallon tank water heater. The first hour rating should be at least 60 gallons to handle morning peak demand.
What is first hour rating (FHR)?
First hour rating is the gallons of hot water a heater can deliver in one hour starting with a full tank. It combines stored capacity with recovery rate.
Tankless vs tank water heater — which is better?
Tankless heaters save energy and provide endless hot water but cost more upfront and may struggle with simultaneous demand. Tanks are cheaper but less efficient and run out of hot water.