Hydronics IQ
Free Hydronic Formula Calculator
Enter any two values to calculate the third: flow (US GPM), temperature difference (ΔT in °F), or heat output (BTU/hr). No account or purchase is required.
Water heat transfer formula
BTU/hr = 500 × US GPM × ΔT (°F)
- Heat output: BTU/hr = 500 × flow × Delta T.
- Temperature difference: Delta T = BTU/hr ÷ (500 × flow).
- Flow: US GPM = BTU/hr ÷ (500 × Delta T).
For example, 5 US GPM with a 20°F temperature difference transfers approximately 50,000 BTU/hr.
The factor of 500 is an approximation for water, not glycol mixtures. This quick estimate does not replace full system design or pump sizing.