How it works
Water carries heat in proportion to how much flows and how much its temperature changes. A gallon of water weighs about 8.33 lb and takes about 1 BTU per pound per degree Fahrenheit; multiplying by 60 minutes per hour gives 500 BTU/hr per gpm per °F.
Rearrange the same equation to find the flow a given load needs, or the temperature drop you can expect across a coil, zone, or radiator at a given flow.
Worked example
10 gpm of water with a 20°F drop through the load:
- Heat flow = 500 × 10 gpm × 20 °F = 100,000 BTU/hr
- Same load in watts = 100,000 × 0.29307 = 29,307 W
| Input | Value |
|---|---|
| Find | Heat flow (BTU/hr) |
| Heat flow | 100000 BTU/hr |
| Flow rate | 10 gpm |
| Temperature drop (supply minus return) | 20 °F |
| Result | Value |
|---|---|
| Heat flow | 100,000 BTU/hr |
| Flow rate | 10 gpm |
| Temperature drop | 20 °F |
Assumptions and limits
- Plain water near room temperature. The 500 factor drops with hot water and glycol antifreeze mixes, which carry less heat per gallon.
- Sensible heat only; no phase change.
- The temperature difference is measured across the load, not against room temperature.
Common questions
Where does the 500 come from?
8.33 lb per gallon × 60 minutes per hour × 1 BTU/lb·°F ≈ 500.
How do I size the flow for a heating load?
Divide the BTU/hr by 500 times the design temperature drop. A 60,000 BTU/hr load at 20°F needs 6 gpm.
Does it work with antifreeze?
Only roughly; glycol mixes carry less heat per gallon, so the constant is lower and flow must be higher.
What is a typical ΔT?
It depends on the system design; this calculator does not choose it for you. Use the manufacturer or design guidance for your emitters.
Sources
- Sensible heat of water: Q = 500 × gpm × ΔT, from 8.33 lb/gal × 60 min/hr × specific heat of about 1 BTU/lb·°F.
Updated 2026-09-30