01
Formula
Gallons per hour = BTU/h × efficiency ÷ (8.34 × rise).
First hour = storage gallons + recovery gallons. Efficiency is the percent you type, divided by 100 inside the formula.
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Water heater sizing calculator for recovery and storage
This water heater sizing calculator estimates recovery from input BTU, efficiency, and the temperature rise, then adds storage for a first-hour figure. Efficiency and the rise are inputs. It is not a fixture-unit sizing under the plumbing code.
First-hour estimate
88gal
Recovery47.96 gal/h
Storage40 gal
Rise70 °F
Planning estimate from the numbers you enter. Coverage, density, waste and price are assumptions, not a supplier quote or an engineering specification.
How it works
Recovery uses 8.34 pounds per gallon and the degrees you add.
Gallons per hour equal input BTU per hour times efficiency, divided by 8.34 times the temperature rise. A 40,000 BTU input at 70 percent efficiency and a 70 degree rise recovers 47.96 gallons per hour. A 40 gallon tank plus that recovery is a first-hour estimate of 87.96 gallons. Electric units are often near 100 percent. Gas units are lower. Type the efficiency of the heater you are pricing. Incoming water temperature is already inside the rise: outlet minus inlet. This water heater sizing calculator does not count showers or a dishwasher. Compare the first-hour number with the peak hour you measured.
02
Example
40,000 BTU/h, 70 percent, 70 degree rise, 40 gallon tank.
Recovery is 47.96 gallons per hour. The first-hour estimate is 87.96 gallons.
03
Tips
A hotter rise lowers the gallons.
The same burner makes fewer gallons when it has to add more degrees. Do not keep a 70 degree rise if the inlet water is colder than that assumption.
04
Equipment
The heater is the equipment.
Dengaro arranges financing for a water heater. Gallons are not a loan amount.
Important questions
Know what to expect.
Is the first hour a code calculation?+
No. It is storage plus one hour of recovery. A plumbing code first-hour rating is tested. Use the published rating when you have it.
What efficiency should gas use?+
Whatever the specification says. Seventy percent is a planning default, not a rating.
Does this include a recirculation loop?+
No. A loop adds load. This water heater sizing calculator is a single rise and a single tank.
Where do BTU come from for an electric unit?+
Watts times 3.412. The BTU calculator can make that conversion. Then type the BTU here.
Financing built into the sale