Calculate how fast a water heater recovers hot water from its BTU input, efficiency, tank size and temperature rise. Get gallons-per-hour recovery and full-tank recovery time.
After the showers drain the tank, the burner goes to work. Recovery rate - gallons per hour reheated - decides whether the third shower arrives hot or acceptable.
Water Heater Recovery Rate Calculator turns input rating, efficiency and temperature rise into a real recovery number.
Example - 40 gal, 40,000 BTU/h gas, 80%. At a 70 F rise each gallon costs ~583 BTU, so recovery is 40,000 x 0.8 / 583 = ~55 GPH. A 40 gal drawdown rebuilds in under an hour - enough to keep a four-person morning moving.