Convection Heat Transfer Calculator (Newton’s cooling)

Understanding the convection heat transfer calculator — concept and principle

Convection is heat transfer between a surface and a moving fluid such as air or water. This calculator uses Newton's law of cooling in its rate form to find the heat flow from a hot pipe, radiator, electronic heat sink or human body to the surrounding fluid, or from a warm fluid into a cooler surface.

Convection heat = h × area × temperature gap: 25 × 2 × 40 = 2,000 W here.

How it is calculated

Q = h · A · (T_s − T_∞)

h: convective heat transfer coefficient in W/m²K. It is not a material property; it depends on the fluid, its speed, the geometry and the flow regime.
A: surface area in contact with the fluid (m²).
T_s: surface temperature (°C).
T_∞: fluid temperature far from the surface (°C).

A positive result means heat flows from the surface into the fluid; a negative one means the fluid is heating the surface.

The principle

Right at the wall the fluid is stationary, so heat crosses a thin boundary layer by conduction and is then carried away by the moving fluid. The thinner that layer, the steeper the temperature gradient and the larger the heat flow. Instead of solving the flow in detail, engineers lump all of this into the single coefficient h, so heat flow becomes simply proportional to area and temperature difference. Values of h come from experiments and correlations using dimensionless numbers (Nusselt, Reynolds, Prandtl, Grashof).

Worked example

Inputs: h = 25 W/m²K, A = 2 m², T_s = 60 °C, T_∞ = 20 °C.
Temperature difference: 60 − 20 = 40 K.
Q = 25 × 2 × 40 = 2,000 W.

That is 2 kW, roughly the output of a large domestic radiator, but note that h = 25 is at the top of the natural-convection range for air.

Each point is the calculator re-run with only “Surface (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Surface (°C)	Heat flow (W)
15	-250
30	500
45	1,250
60	2,000
75	2,750
90	3,500
120	5,000

How to read the result

The result is very sensitive to h, so choose it carefully. Rough ranges: free convection in air about 5–25 W/m²K, forced convection in air about 10–200, liquid water from a few hundred to around 10,000, and boiling or condensation even higher. A fan or pump raises h and is the usual way to increase cooling. Remember that a surface in air also loses heat by radiation, which can be comparable to natural convection.

Common mistakes

Using a "typical" h without checking whether the flow is natural or forced.
Using the wrong area (for a pipe, the outer circumference times the length).
Forgetting radiation for hot surfaces in still air.
Assuming h stays constant as temperatures or speeds change.

Keep in mind: h values are approximate and can be uncertain by 25% or more even from good correlations. For equipment sizing or safety-critical cooling, use validated correlations or test data and have the design checked by a qualified engineer.

Frequently asked questions

Is this the same as Newton's law of cooling?

Yes, this is its rate form. The time-dependent cooling curve follows when you combine it with the object's heat capacity.

How can I estimate h for my case?

Compute the Reynolds or Rayleigh number, choose a matching Nusselt correlation from a heat-transfer textbook, and use h = Nu·k/L.

Sources: Incropera, DeWitt, Bergman and Lavine, Fundamentals of Heat and Mass Transfer, Wiley · Çengel and Ghajar, Heat and Mass Transfer: Fundamentals and Applications, McGraw-Hill

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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