Heat Transfer Coefficient Calculator
Convective heat transfer from a surface — Newton's law of cooling — with the coefficient solved in whichever direction you need.
How fast a surface loses heat to moving air or water.
How the heat transfer coefficient calculator works
Newton's law of cooling: Q̇ = hAΔT, where h is the convective heat transfer coefficient. Unlike conductivity, h is not a material property — it depends on the fluid, the geometry and above all the flow speed.
Typical values span four orders of magnitude: 5–25 W/m²·K for still air, 10–200 for forced air, 500–10,000 for water, and up to 100,000 for boiling. That range is why a fan matters so much.
Formula: Q̇ = h A ΔT
Worked examples
| Inputs | Result | Note |
|---|---|---|
| Natural convection in air | 1,500 | 1,500 W |
| Find h from a measured heat flow | 25 | h = 25 |
| Water cooling | 30,000 | 30 kW from half a square metre |
FAQFrequently asked questions
What is the heat transfer coefficient?
How readily heat moves between a surface and a fluid, in watts per square metre per kelvin. It is not a material property — it depends on the flow.
What are typical values?
Still air 5–25, forced air 10–200, water 500–10,000, boiling up to 100,000 W/m²·K.
Why does a fan help so much?
Because forcing the air can raise h by a factor of ten. The thin stagnant layer at the surface is the bottleneck, and moving air thins it.
How is this different from conduction?
Conduction moves heat through a solid and depends on the material; convection moves it into a fluid and depends on how that fluid flows.
Why is water so much better than air?
Higher density and specific heat, so a given volume carries far more energy away. It is why engines and data centres move to liquid cooling.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Physical Laboratory — the UK's national measurement institute
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.