Heating Power Calculator
Enter flow and ΔT to get thermal power in kW, BTU/h and tons — with the hydronic shortcut (kW ≈ L/min × ΔT ÷ 14.3) built in.
Last updated: 2026-09-28
How the calculation works
- Mass flow (flow × density) times specific heat times ΔT gives kilowatts directly.
- Glycol mixtures carry less heat per litre — the fluid selector applies their real c and ρ.
- BTU/h and tons conversions connect to US equipment ratings.
Formula
Q = ṁ · c · ΔT (ṁ = flow × density)
| Symbol | Meaning | Unit |
|---|---|---|
Q | Thermal power | kW |
ṁ | Mass flow | kg/s |
Worked example
Interpreting the result
This equation runs both directions: pick two of (flow, ΔT, power) and the third follows. A chiller delivering 20 kW at 5 °C design ΔT needs 57 L/min — undersized flow shows up as a bigger ΔT and poor heat exchange. Glycol costs capacity: a 50% mix needs ~20% more flow for the same duty, which is why glycol systems upsize pumps as well as checking freeze protection.
Assumptions
- Steady flow, no phase change, fluid properties at operating temperature.
- Densities: water 998, 30% glycol 1038, 50% glycol 1062, oil 870 kg/m³.
Limitations
- Steam systems (phase change) need latent-heat math, not this sensible model.
- Does not size the equipment — it converts measured flow/ΔT into duty (or vice versa).
Frequently asked questions
How do I calculate boiler kW from flow and temperature?
kW = flow(L/min) × ΔT(°C) ÷ 14.3 for water. 20 L/min heated 10 °C ≈ 14 kW — verify by measuring both sides of the boiler.
Why does glycol reduce heating capacity?
Glycol is thicker and carries less heat per litre: a 50% mix has ~26% less heat capacity than water. The same pump and coil deliver proportionally less duty.