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Convective Heat Transfer Calculator

Estimate the rate of heat carried away from (or delivered to) a surface by a moving fluid using Newton's law of cooling. Enter the convective heat transfer coefficient, the surface area and the surface-to-fluid temperature difference to get the convective heat flow and the heat flux.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

How to use this calculator

  1. Enter the convective heat transfer coefficient h in W/m2.K for your surface and fluid conditions.
  2. Enter the surface area A in square metres and the surface-to-fluid temperature difference dT in kelvin (a 1 K difference equals a 1 degC difference).
  3. Read off the convective heat flow Q in watts and the heat flux q in watts per square metre.

How it works

Newton's law of cooling states Q = h * A * dT, where h is the convective heat transfer coefficient, A is the wetted surface area and dT is the temperature difference between the surface and the bulk fluid. The heat flux (per unit area) is q = h * dT. The coefficient h bundles together all the complex fluid-dynamics of the boundary layer into a single number, so its accuracy depends entirely on choosing an h that suits your flow regime and geometry.

Worked example

Worked example. For a surface with h = 25 W/m2.K, area A = 2 m2 and a temperature difference dT = 30 K: q = 25 * 30 = 750 W/m2 and Q = 25 * 2 * 30 = 1,500 W.

Common mistakes

  • Using a guessed h value: h can range from a few W/m2.K for still air to thousands for boiling or forced liquid flow, so a poor estimate dominates the error.
  • Mixing up the temperature difference: dT is the surface-to-fluid difference, not the inlet-to-outlet change of the fluid.
  • Using the wrong area: use the actual wetted convective surface area, not the frontal or projected area.

Frequently asked questions

What is a typical value for the convective coefficient h?

Roughly 5-25 W/m2.K for natural convection in air, 10-500 for forced air, 100-15,000 for forced water, and much higher for boiling or condensation. Always pick a value (or Nusselt-number correlation) that matches your flow.

Do I use kelvin or degrees Celsius for dT?

Either works because a temperature difference has the same magnitude in kelvin and degrees Celsius; a 30 K difference is a 30 degC difference.

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