First Hour Rating (FHR) Calculator
Calculate the First Hour Rating you need for your water heater based on your household's peak hour usage.
Table of Contents
Peak Hour Usage
Assume 20 gallons per shower.
Assume 20 gallons per bath.
Assume 14 gallons per load.
Assume 30 gallons per load.
Required First Hour Rating
What is First Hour Rating (FHR)?
The FHR is the amount of hot water in gallons a heater can supply per hour (starting with a tank full of hot water). It depends on tank capacity, source of heat, and the size of the burner or element.
How to Use This Calculator
To determine the right First Hour Rating for your home, you need to estimate your peak hour demand. The peak hour is the busiest one-hour period of the day for hot water usage in your home (usually in the morning).
- Showers/Baths: Count the number of showers and baths taken consecutively or simultaneously during the peak hour.
- Appliances: If you typically run the dishwasher or washing machine during this same hour, include those loads.
The calculator multiplies these activities by average water usage estimates to give you a target FHR. When shopping for a tank water heater, look for a model with an FHR that matches or slightly exceeds your calculated peak hour demand.
First Hour Rating (FHR) Sizing
FHR measures the total volume of hot water a storage water heater can deliver in a single hour of peak demand, combining the usable preheated tank volume with the hourly heating recovery rate.
Formula Used
First Hour Rating = Tank Capacity * 0.70 + Hourly Recovery Rate (GPH)
Reference Quick Chart
| Household Members | Typical Bathrooms | FHR Sizing Target |
|---|---|---|
| 1 - 2 People | 1.0 Bathroom | 45 - 55 Gallons |
| 3 - 4 People | 1.5 - 2.0 Bathrooms | 60 - 75 Gallons |
| 5+ People | 2.5+ Bathrooms | 80+ Gallons |
Recommended Water Heater Products
Tankless Water Heaters
Save up to 50% on energy costs with an endless supply of hot water. Best for active, multi-shower households looking to cut utility bills.
Storage Tank Water Heaters
Reliable and affordable upfront solutions. Delivers high peak-hour volume to ensure you never run out of hot water during busy mornings.