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Convective Heat Transfer Rate Calculator

Enter the heat transfer coefficient, surface area and temperature difference.

How it is calculated

The tool applies Newton's law of cooling: Q = h × A × ΔT. Here h is the convection coefficient in W/(m²·K), A is the wetted surface area in m², and ΔT is the surface-to-fluid temperature difference in K (a difference of 1 K equals 1 °C). The result Q is in watts and is also shown in kW (Q ÷ 1000), together with the heat flux q = h × ΔT in W/m². Area must be greater than zero; h and ΔT are not restricted, so a negative ΔT gives a negative Q, meaning heat flows into the surface instead of out of it. The tool does not estimate h for you.

Worked example

Take a panel with h = 25 W/(m²·K), A = 1.5 m² and ΔT = 40 K. First the flux: 25 × 40 = 1,000 W/m². Then multiply by the area: 1,000 × 1.5 = 1,500 W. Result: 1,500.00 W, which the tool also shows as 1.500 kW and a flux of 1,000.00 W/m².

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Common mistakes

  • Entering area in cm² or mm² instead of m². The coefficient is per square metre, so a 1.5 m² panel typed as 15000 cm² gives a result 10,000 times too large.
  • Using the surface temperature or the fluid temperature alone instead of their difference. ΔT must be surface minus fluid.
  • Guessing h. It depends on the fluid, velocity and geometry, and values for still air and for flowing water differ by orders of magnitude, so the answer is only as good as h.
  • Using the wrong sign: if the fluid is hotter than the surface, ΔT is negative and the negative Q is correct, not an error.
  • Applying the result to a surface that also radiates strongly; this formula covers convection only and ignores radiation and conduction.

About this calculator

Newton's law of cooling describes how heat transfers between a solid surface and a moving fluid (air, water, or another fluid) flowing past it — the process known as convective heat transfer. The rate of heat transfer, Q, is proportional to the convective heat transfer coefficient (h), which depends on the fluid's properties and flow conditions, the surface area (A) in contact with the fluid, and the temperature difference (ΔT) between the surface and the fluid: Q = h·A·ΔT.

This calculator computes Q directly from those three inputs. It's a foundational formula in thermal and mechanical engineering, used to size heat sinks and radiators, estimate heat loss from pipes and building surfaces, and analyze cooling systems for electronics — anywhere a designer needs to know how quickly heat will move off a surface into a surrounding fluid.

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Comments (1)

  • Editorial team

    The sign matters. With h of 10, an area of 1 and a temperature difference of -5, the tool returns -50.00 W, meaning heat flows into the surface, not out of it. Pick one convention for the difference and keep it. Also remember h varies a lot with air speed, so a single value is an estimate, not a measurement. Which surface and fluid are you modelling?

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