About this calculatorThis calculator determines the hydronic flow rate (GPM) needed to transport a given heat load at a specified temperature difference. Use it when sizing piping, pumps, or control valves for hot water or chilled water systems. The standard formula uses the constant 500, which accounts for the density and specific heat of water at typical HVAC operating temperatures.
Use whenYou need to size hydronic piping or pumps based on a heating or cooling load.
FormulaGPM = BTU/Hr ÷ (500 × ΔT)
Variables
Heat Load UnitsBTU/Hr or kWHeat LoadHeat load valueΔTTemperature difference in °FFluid TypeWater, 25% glycol, or 50% glycol
BTU/hr
Unit system for heat load input
Heating or cooling load value
Supply-to-return temperature difference
Water
Hydronic fluid used in the system
Flow Rate (GPM)—
How is hydronic flow rate calculated from heat load?
Hydronic flow rate is the volume of water (in gallons per minute) required to transport a given heat load through a piping system at a specified temperature difference. The formula is: GPM = BTU/hr ÷ (500 × ΔT), where ΔT is the temperature difference between the supply and return water in °F. The constant 500 is derived from the density of water (8.33 lb/gal), the specific heat of water (1.0 BTU/lb-°F), and 60 minutes per hour: 8.33 × 1.0 × 60 = 500. This constant applies at typical HVAC operating temperatures (40–200°F); for glycol mixtures or extreme temperatures, the actual fluid properties should be used. Standard design ΔT for chilled water systems is 10–14°F, and for hot water heating systems it is 20–40°F. A larger ΔT reduces the required flow rate and pipe size but increases the coil size needed to achieve the heat transfer.
