Convection Heat Transfer Calculator
Heat carried away from a surface by air or water, from Newton's law of cooling, with the area or coefficient you would need.
Results
What this tool does
Convection is heat leaving a surface into a fluid that carries it away, and Newton's law of cooling says the rate is simply proportional to how much hotter the surface is than the fluid. All the difficulty lives in one number — the heat transfer coefficient — which is not a material property but a description of the whole situation: what is flowing, how fast, over what shape. Give it, along with the area and the two temperatures, and this page returns the heat flow, the flux per square metre, and the area or coefficient you would need to shed a kilowatt.
Formula
Newton's law of cooling: Q = h × A × (T surface − T fluid)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
h | Heat transfer coefficient | W/(m²·K) |
a | Area | m² |
ts | Surface temperature | °C |
tf | Temperature of the air or water | °C |
Q | Heat crossing it | W |
QA | Heat flux | W/m² |
DT | Temperature difference | K |
R | Thermal resistance | K/W |
UA | Conductance of the surface | W/K |
Q10 | Heat flow at a 10 K difference | W |
E | Energy in one hour | kWh |
Worked example
- Heat transfer coefficient25 W/(m²·K)
- Area2 m²
- Surface temperature80 °C
- Temperature of the air or water20 °C
- Heat crossing it3000.0000 W
- Heat flux1500.0000 W/m²
- Temperature difference60.0000 K
- Thermal resistance0.020000 K/W
- Conductance of the surface50.000000 W/K
- Heat flow at a 10 K difference500.0000 W
- Energy in one hour3.000000 kWh
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
Frequently asked questions
Where does the coefficient come from?
From correlations for your specific geometry and flow, and it is by far the weakest number in the calculation. Unlike conductivity, it is not a property of a material — it describes a whole situation: what fluid, moving how fast, over what shape, in which orientation. Still air against a vertical wall gives something around 5 to 10; air blown across a surface, 25 to 250; water, hundreds to thousands; boiling water, tens of thousands. Those ranges are not precision. Anyone who needs a real answer looks up the Nusselt correlation for their exact case, and even then expects to be twenty per cent out.
Does this include radiation?
No, and at higher temperatures that matters a great deal. Newton's law of cooling describes only the heat carried away by moving fluid. A surface also radiates, and because radiation goes with the fourth power of absolute temperature it starts small and then overtakes convection. For a radiator at sixty degrees in a room the two are roughly comparable; for a furnace wall, radiation dominates completely. Add the two together using the Stefan-Boltzmann calculator when the surface is hot.