What is recovery rate?

Recovery rate is the volume of water (gallons or liters) a heater can raise from inlet temperature to the set point in one hour. It depends on heat input and the temperature rise required.

What is first-hour delivery?

First-hour delivery (FHD) estimates how much hot water you can get in the first hour starting from a full, hot tank. It equals 70% of the tank capacity (already hot) plus one full hour of recovery output.

Why 70% for the tank portion of FHD?

Hot and cold water mix inside the tank as you draw hot water. In practice, only about 70% of the rated tank volume is usable before the outlet temperature drops unacceptably. The DOE Energy Guide label uses this same factor.

How is reheat time calculated?

Reheat time = (tank volume × weight per unit × ΔT) / heat input. For US units: (gallons × 8.33 lb/gal × ΔT°F) / BTU/hr gives hours. For metric: (liters × ΔT°C) / (kW × 860 kcal/kWh) gives hours.

What BTU/hr value should I use?

Use the "Input" or "Heater Input" rating from the heater nameplate, not the "EF" or efficiency-adjusted output. Common residential gas heaters range from 32,000–100,000 BTU/hr; electric elements typically 4,500–9,000 W (convert: 1 kW = 3,412 BTU/hr).

Can I convert kW to BTU/hr?

1 kW = 3,412 BTU/hr. Switch to Metric mode if your heater is rated in kW — the calculator handles both unit systems natively.

Hot Water Heater Recovery Rate Calculator

Enter your water heater input power, tank volume, and temperature rise to get recovery rate, first-hour delivery, and reheat time.

Heater Specifications

Unit System

Heater Input Rating

BTU/hr

Tank Volume

gallons

Inlet (Cold) Water Temperature

°F

Target (Hot) Water Temperature

°F

Results

Enter your heater specs and click Calculate.

Summary

This calculator computes the recovery rate of a water heater — how many gallons (or liters) per hour it can heat from the cold inlet temperature to the target output temperature. It also shows first-hour delivery (FHD), which combines the usable stored volume with one hour of recovery, and the time required to fully reheat a drained tank. Both US (BTU/hr, gallons, °F) and metric (kW, liters, °C) unit systems are supported.

How it works

Select your unit system: US (BTU/gal/°F) or Metric (kW/L/°C).
Enter the heater input rating from the nameplate (BTU/hr or kW).
Enter the tank volume in gallons or liters.
Enter the cold inlet water temperature and the target hot water temperature.
The calculator instantly shows recovery rate, first-hour delivery, and full-tank reheat time.

Use cases

Size a replacement water heater for a household or small business.
Estimate whether your current heater can supply back-to-back showers.
Compare electric vs. gas heater performance before purchasing.
Troubleshoot slow hot water recovery after a heater service.
Verify a plumbing specification against manufacturer recovery claims.
Calculate reheat time for a commercial or industrial tank heater.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu