Hot Water Heater Sizing Calculator

Estimate the required water heater capacity based on demand and temperature rise.

Calculator

Hot Water Demand (GPH)

Incoming Water Temperature (°F)

Target Water Temperature (°F)

Fuel Type

Required Capacity

0 BTU/hr

Enter values above and click calculate.

How It Works

This calculator estimates the minimum BTU output required to meet your hot water demand based on occupancy, usage, and the temperature rise needed from your incoming supply.

BTU/hr = (GPH × 8.33 × ΔT) ÷ Efficiency

Where:
Hot Water Demand (GPH) = gallons per hour of hot water demand
8.33 = weight of one US gallon of water in pounds
ΔT = temperature rise required (Target Water Temperature - Incoming Water Temperature) (°F)
Efficiency = rated efficiency of the unit - gas/propate tank: 80%, electric tank: 92%, tankless units: 96-99%
Result = minimum BTU/hr output your water heater must deliver

Rough residential demand estimates — roughly 20 GPH per person. Add separately for dishwashers (3 GPH), washing machines (5 GPH), and commercial equipment.

A household with 4 people at 20 GPH each with a 70°F temperature rise requires approximately 46,648 BTU/hr of heating output.