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HVAC made simple
WATER HEATER CALCULATOR
First Hour Rating Calculator
Estimate how many gallons of hot water a storage water heater may be able to supply during a high-demand first hour.
1. Enter Water Heater Tank Size
Start with the nominal storage capacity of your water heater.
gallons
%
This is a simplified assumption. Actual usable hot-water
delivery varies with tank design, thermostat settings,
incoming-water temperature, draw pattern and mixing.
Common Tank Sizes
2. Select the Recovery Method
Choose how you want the calculator to estimate how much additional hot water the heater can produce during the hour.
Gas Water Heater Information
Enter the burner input and an assumed thermal efficiency.
BTU/hr
%
For a specific product, use manufacturer performance
information rather than relying on a generic assumption.
Electric Water Heater Information
Enter the power of the heating element that is actively contributing to recovery.
kW
Many conventional dual-element residential water heaters
operate one element at a time. Do not automatically add
both element ratings unless the specific heater is designed
for simultaneous operation.
%
Published Recovery Rate
Enter the manufacturer’s recovery rate directly.
gal/hr
Make sure the published recovery rate is based on the same
or a comparable temperature-rise condition.
3. Enter the Temperature Rise
Recovery depends heavily on how many degrees the incoming water must be heated.
°F
°F
Temperature Rise
70°F
120°F − 50°F = 70°F
ESTIMATED FIRST HOUR RATING
Your Water Heater FHR Estimate
Estimated First Hour Rating
90 gallons
Approximate hot-water delivery during a high-demand first hour
Tank Capacity
50 gal
Usable Stored Water
35 gal
Estimated Recovery
55 gal/hr
Temperature Rise
70°F
How the Estimate Is Built
Usable Stored Hot Water
35 gallons
+
First-Hour Recovery
55 gallons
=
Estimated FHR
90 gallons
Estimated Recovery Rate
55.0 gallons/hourWhat Does This Result Mean?
Manufacturer FHR Is the Number to Use When Comparing Actual Water Heaters
This calculator produces an educational estimate. The official First Hour Rating for a specific storage water heater should come from the manufacturer’s product information and applicable standardized test data. Tank design, controls, burner or element operation, thermostat behavior, draw pattern and other factors can cause actual tested FHR to differ from a simplified estimate.
What Is First Hour Rating?
First Hour Rating, commonly abbreviated as FHR, describes how much hot water a storage water heater can deliver during a high-demand first hour when starting with a heated tank. It is different from tank capacity because the heater begins reheating incoming cold water as hot water is drawn from the tank.
This is why two 50-gallon water heaters can have different first-hour performance. A heater with faster recovery may provide substantially more usable hot water during the first hour even though both products have similar storage capacities.
Example: 50-Gallon Gas Water Heater
Suppose a 50-gallon storage water heater is assumed to have 70% immediately usable stored hot water. That represents about 35 gallons of usable stored water. If the heater can recover approximately 55 gallons during the hour, the simplified estimate would be:
35 gallons
+
55 gallons
=
90-gallon estimated FHR
This example demonstrates why tank size and First Hour Rating should not be treated as the same measurement.
⚠️ Tank Capacity ≠ First Hour Rating
A 50-gallon water heater does not necessarily have a 50-gallon First Hour Rating. FHR reflects both stored hot water and the heater’s ability to recover while water is being used. When choosing a storage water heater, compare the manufacturer’s published FHR with your household’s expected peak-hour hot-water demand.
Important:
This calculator provides an estimated indication for educational
purposes only. It does not reproduce an official standardized
First Hour Rating test. Actual FHR depends on water-heater design,
nominal and usable storage volume, burner or element operation,
controls, incoming-water temperature, thermostat setting, draw
pattern, recovery performance and other conditions. For the most
accurate information, use the manufacturer’s published First Hour
Rating for the exact model and consult a qualified plumbing or HVAC
professional when selecting or installing a water heater.