Water Heater Size Calculator
The right water heater size ensures you never run out of hot water without wasting energy on an oversized unit. This calculator estimates the required capacity for both tank and tankless systems.
Enter Your Measurements
Results
How to Use This Calculator
- Enter the number of people in your household.
- Enter peak simultaneous hot water usage (e.g., 2 showers at once).
- Enter your incoming cold water temperature (50°F is average).
- Enter desired output temperature (120°F is standard).
- Select tank or tankless to see the relevant sizing.
How Water Heater Calculation Works
For tank water heaters, sizing is based on First Hour Rating (FHR) — how many gallons the tank can deliver in the first hour of use. This depends on tank size and recovery rate. A household of 4 typically needs a 50-gallon tank with 60+ gallon FHR. For tankless units, sizing is based on the maximum simultaneous flow rate (GPM) and the temperature rise needed. A tankless unit rated for 5 GPM at a 50°F rise can handle 2 showers simultaneously in most regions.
Formula
Peak demand = 12 gal/person (first hour). Tank FHR = Peak demand (round up to standard sizes). Tankless GPM = simultaneous fixtures. Tankless BTU = GPM × 500 × ΔT.