BTU Calculator

Sizing Heating Equipment in BTU/hr

BTU per hour remains the standard unit for sizing boilers, water heaters, and heat exchangers in much of the industry. This calculator applies the standard water-heating load formula used across HVAC and combustion equipment sizing.

The Formula

BTU/hr = Water Flow(GPM) × 500 × Temperature Rise(°F)

500 combines water's density (8.33 lb/gal), specific heat (1 BTU/lb.°F), and 60 minutes per hour into one constant.

Worked Example

A system circulating 20 GPM with a 30°F temperature rise:

BTU/hr = 20 × 500 × 30 = 300,000 BTU/hr

How to Use This Calculator

Enter the Water Flow Rate in GPM.
Enter the required Temperature Rise in °F.
Select Calculate to get the heating load in BTU/hr.

Related Calculations

Need to convert this to other energy units? See the Energy Conversion Calculator, or size the boiler fuel input in the Boiler Fuel Requirement Calculator.