Water Heater Size Calculator

Calculate the right water heater tank size based on household size and hot water usage patterns.

Results

Visualization

How It Works

Water heater sizing is based on First Hour Rating (FHR) — the gallons a heater can deliver in the first hour of use. A family of 4 with average usage needs about 50-65 gallons.

The Formula

Peak Demand = People x 12 gal/person x Usage Factor x 1.5
FHR = Tank Capacity x 0.7 + 20

Variables

  • 12 gal — Average hot water per person per peak hour
  • FHR — First Hour Rating — gallons delivered in first hour

Worked Example

Family of 4, medium use: 4 x 12 x 1.0 = 48 gal/day. Peak demand = 72 gal. Recommended: 65-gallon tank.

Practical Tips

  • Match the FHR to your peak demand, not the tank size alone
  • Tankless heaters eliminate standby loss but cost more upfront
  • Heat pump water heaters save 50-60% on energy but need warm ambient air

Last updated: March 11, 2026 · Reviewed by the PlumbCalcs Editorial Team

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