Heat Duty Calculator (Q = ṁ·cp·ΔT)
By Santaji GadepatilinUpdated October 27, 2026
Heat duty (kW) vs Temperature difference (in − out) (K / °C) — other inputs held at your current values.
How to use this calculator
Enter the mass flow, specific heat and the temperature difference between inlet and outlet. The heat duty is calculated in kW (or hp). You can also find the flow needed to carry a given load, or the temperature rise for a known heater power.
The formula
Q̇ = ṁ × cp × ΔT
Worked example
2 kg/s of water cooled by 10 °C in a heat exchanger:
Q̇ = 2 × 4.186 × 10 = 83.7 kW
Typical specific heats
| Fluid | cp (kJ/kg·K) |
|---|---|
| Water | 4.186 |
| Glycol 30% | ~3.7 |
| Thermal oil | ~2.0 – 2.5 |
| Air | 1.005 |
| Steel | 0.49 |
Practical uses
- Chiller and boiler sizing — enter design flow and ΔT.
- Energy metering — BTU/energy meters calculate exactly this using a flow meter and two temperature sensors (often matched Pt100 or Pt500 pairs).
- Checking performance — compare measured duty with the design value to spot fouling.
Frequently asked questions
What is heat duty?
The rate at which heat is added to or removed from a fluid, usually in kW. For a heat exchanger, it is the same on both the hot and cold sides, ignoring losses.
What specific heat should I use for water?
4.186 kJ/kg·K is standard. It changes only slightly between 0 and 100 °C.
Can I use volumetric flow?
Convert it to mass flow first by multiplying by density. Use the mass flow rate calculator.