SettingsSettings for this calculationUS
The rated storage capacity of your water heater, in the unit shown.
Check the tank's data plate — this is usually printed prominently near the top.
The temperature of the cold water feeding the tank.
Groundwater temperature varies by season and region — colder incoming water means a longer recovery time for the same target temperature.
The thermostat setting on the water heater.
120°F (49°C) is a common default balancing scald safety and hot water demand.
The heater's rated power input, from its data plate, in the unit shown.
Standard residential gas water heaters are commonly 30,000–40,000 BTU/hr (about 9–12 kW input); electric heaters state watts directly.
How much of the input energy actually ends up in the water.
This depends entirely on what is heating the tank. A gas storage heater loses about a quarter up the flue, so 0.75 is the usual figure and the default here. An electric immersion element sits IN the water and puts essentially all of its input there, so use 0.95 to 1.0. An indirect coil fed from a boiler is also close to 1.0 at the coil — the boiler has its own efficiency, but that is a separate question about the fuel bill rather than about how long the cylinder takes. A heat pump is different again and is better sized on its own output figure than on this one.
Recovery time
58.5 minutes
This estimates recovery from a fully-drained tank using the nameplate input rating — real-world recovery is somewhat slower due to standby losses and burner cycling.
- Recovery rate
- 0.86 gal/min
They open the calculator with your figures already in it
Water Heater Recovery Time Calculator: 58.48 minutes — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard recovery formula: recovery rate = (power input × recovery efficiency) ÷ (8.33 BTU/gal·°F × temperature rise); recovery time = tank capacity ÷ recovery rate. Efficiency is 0.75 for a gas storage heater, near 1.0 for an immersion element or an indirect coil. Metric inputs convert at the formula's boundary.
Inputs used
- Tank Capacity
- 50 gal
- Incoming Water Temperature
- 50 °F
- Target Water Temperature
- 120.2 °F
- Burner/Element Power Input
- 40000.54 BTU/hr
- Recovery efficiency
- 0.75
Intermediate steps
- Recovery rate
- 0.86 gal/min
Confidence note: This estimates recovery from a fully-drained tank using the nameplate input rating — real-world recovery is somewhat slower due to standby losses and burner cycling.
What this calculation does not cover
- This is the time to reheat the entire tank contents from your incoming temperature up to the thermostat setting. It is not the time until you have usable hot water again — a tank stratifies, so the top of the cylinder is back up to temperature well before the whole volume is.
- It assumes nothing is drawn off while the tank recovers. Any continuing draw replaces heated water with cold and pushes the time out; a draw that outruns the recovery rate means the tank never catches up at all.
- Nameplate input is taken at face value, with no allowance for gas derating at altitude, an electric element running below its rated supply voltage, or scale on the element and fouling on the flue side of an older heater. Most residential dual-element electric heaters energise only one element at a time, so enter one element's rating rather than the sum of both.
- Recovery efficiency stops at 1.0, so this cannot represent a heat pump water heater, which moves more heat than it draws in electricity and whose output falls as the surrounding air gets colder. For an indirect coil the figure you enter is the heat reaching the coil — a boiler that cannot sustain that output, or a coil undersized for it, recovers more slowly than this predicts.
- Nothing here checks the target temperature for safety. Stored water temperature is a trade-off between scald risk at the outlet and controlling bacterial growth in the tank, and this calculator will model any setting in its range without comment — set it against the requirements that apply to your installation, and fit a thermostatic mixing valve where outlet temperature has to be limited.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Standard recovery formula: recovery rate = (power input × recovery efficiency) ÷ (8.33 BTU/gal·°F × temperature rise); recovery time = tank capacity ÷ recovery rate. Efficiency is 0.75 for a gas storage heater, near 1.0 for an immersion element or an indirect coil. Metric inputs convert at the formula's boundary.
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