Convection Heat Transfer Calculator
Calculate the convective heat-transfer rate q = h · A · ΔT between a surface and a moving fluid. Enter the heat-transfer coefficient, surface area and temperature difference to get results in watts, kW, BTU/h and more.
Find the Heat Transferred Between a Surface and a Moving Fluid
Convection Heat Transfer Formula
Where:
- \(q\) — convective heat-transfer rate (W)
- \(h\) — convective heat-transfer coefficient (W/(m²·K))
- \(A\) — surface area in contact with the fluid (m²)
- \(\Delta T = T_{\text{surface}} - T_{\text{fluid}}\) — temperature difference (K)
This is Newton's law of cooling: the rate at which heat crosses a surface by convection grows in direct proportion to the contact area, the temperature difference between the surface and the fluid, and the coefficient \(h\), which captures how effectively the moving fluid carries heat away.
Convection Heat Transfer – Calculation Example
Example: a 1 m² metal wall at 60 °C is cooled by forced air at 20 °C, giving a heat-transfer coefficient of \(h = 25\) W/(m²·K).
\[ q = h \, A \, \Delta T = 25 \ \tfrac{\text{W}}{\text{m}^2\cdot\text{K}} \times 1\ \text{m}^2 \times (60 - 20)\ \text{K} \]
\[ q = 1000\ \text{W} = 1\ \text{kW} \approx 3412\ \tfrac{\text{BTU}}{\text{h}} \]
Result: the surface transfers 1000 W of heat to the air. The heat flux is \(q/A = 1000\) W/m², and if conditions stay steady for one hour, 3.6 kWh of energy is transferred.
How to Use the Calculator
- Enter the Convective heat-transfer coefficient (h) and pick its unit: W/(m²·K), kcal/(h·m²·°C) or BTU/(h·ft²·°F).
- Enter the Surface area (A) in contact with the fluid, in m², cm², ft² or in².
- Enter the Temperature difference (ΔT) between the surface and the fluid, in °C, K or °F. Enter it as surface temperature minus fluid temperature: positive if the surface is hotter, negative if the fluid is hotter.
- Read the convective heat-transfer rate in watts, kilowatts, BTU/h, cal/s and hp, plus the heat flux and the direction of heat flow in the details table.
What Is Convection Heat Transfer?
Convection is heat transfer between a solid surface and a fluid (air, water, oil or any gas or liquid) that moves over it. The fluid molecules touching the hot surface pick up energy and carry it away as they move, while cooler fluid replaces them. Because the fluid is in motion — driven by a fan or pump (forced convection) or by buoyancy alone (free or natural convection) — convection transfers heat far faster than conduction through stagnant fluid.
The key quantity is the convective heat-transfer coefficient \(h\). It is not a material property like thermal conductivity; it bundles together the fluid's properties (viscosity, density, specific heat), the flow speed, and the geometry and orientation of the surface. This is why \(h\) spans several orders of magnitude: roughly 5–25 W/(m²·K) for air moving by natural convection, 25–250 for forced air, and 100–20,000 for flowing water or condensing steam.
The result \(q\) is a rate: energy per unit time, in watts. A positive value means the surface loses heat to the fluid (cooling, as with a heat sink or radiator); a negative value means the fluid heats the surface (as in a heater or a hot bath). If you only care about the magnitude, use the absolute value shown in the interpretation box. Doubling any one input — \(h\), \(A\) or \(\Delta T\) — doubles the heat transferred, which is why engineers often fight for more surface area (fins) or higher flow (bigger \(h\)) when they need more cooling.
Typical Values of the Heat-Transfer Coefficient
| Situation | h, W/(m²·K) |
|---|---|
| Air, free (natural) convection | 5–25 |
| Air, forced convection (fan) | 25–250 |
| Water, slow flow in pipes | 500–3,000 |
| Water, turbulent flow | 3,000–20,000 |
| Boiling water / condensing steam | 5,000–100,000 |
Assumptions, Limitations and Practical Tips
- \(h\) must match your situation. The single biggest source of error is using the wrong coefficient. Use the table above as a guide, or a correlation from a heat-transfer textbook for your exact geometry and flow conditions.
- Steady state assumed. The formula gives the instantaneous rate for steady temperatures. If the surface or fluid temperature changes over time, \(q\) changes with it — see our Newton's Law of Cooling calculator for the time-dependent case.
- Uniform \(h\) and \(\Delta T\). In reality both vary along a surface (a boundary layer grows downstream). The result is an average over the area, which is usually good enough for design estimates.
- Convection only. Real surfaces also lose heat by thermal radiation and gain or lose it by conduction. At high temperatures or in still air, radiation can be comparable to convection and should be added separately.
- Temperature differences are interchangeable in °C and K. The calculator converts a ΔT in °F to kelvin automatically; you never need to convert absolute temperatures, only the difference.
- When sizing equipment, add a safety margin: calculated \(h\) values from correlations are often uncertain by 25 % or more.
Related Context
Convection is one of the three modes of heat transfer, alongside conduction and radiation. It appears everywhere in engineering and daily life — from CPU heat sinks and car radiators to home heating and weather patterns. If you are analyzing thermal systems more broadly, you may also find our Carnot Efficiency calculator useful for the theoretical limits of heat engines that ultimately reject heat by convection, or the Linear Thermal Expansion calculator for how those same temperature differences change the size of parts.
Frequently Asked Questions
How do I calculate convective heat transfer by hand?
Multiply the three quantities together: \( q = h \, A \, \Delta T \). For example, a wall of \( A = 2\ \text{m}^2 \) exposed to forced air with \( h = 50\ \tfrac{\text{W}}{\text{m}^2\cdot\text{K}} \) and a surface 15 K hotter than the air transfers \( q = 50 \times 2 \times 15 = 1500\ \text{W} \). Just make sure all inputs are converted to SI units first, or let the calculator do the conversions for you.
How do I convert units like BTU/(h·ft²·°F) to W/(m²·K)?
The conversion factors are: \( 1\ \tfrac{\text{BTU}}{\text{h}\cdot\text{ft}^2\cdot\text{F}} \approx 5.678\ \tfrac{\text{W}}{\text{m}^2\cdot\text{K}} \) and \( 1\ \tfrac{\text{kcal}}{\text{h}\cdot\text{m}^2\cdot\text{C}} \approx 1.163\ \tfrac{\text{W}}{\text{m}^2\cdot\text{K}} \). For the other inputs: \( 1\ \text{ft}^2 = 0.0929\ \text{m}^2 \), \( 1\ \text{in}^2 = 6.45\times10^{-4}\ \text{m}^2 \), and a temperature difference in °F equals \( \tfrac{5}{9} \) of the same difference in K. Never convert absolute temperatures — only the difference.
How do I find the right heat-transfer coefficient h for my situation?
Estimating \( h \) is the hardest part. For rough work, use typical ranges: free air 5–25, forced air 25–250, flowing water 500–20,000 W/(m²·K). For accurate work, use an engineering correlation (Nusselt number relations such as \( h = Nu \, k / L \)) matched to your geometry — flat plate, cylinder, pipe flow — and flow regime. Textbook correlations carry 20–30 % uncertainty, so add a design margin when sizing heatsinks, radiators or HVAC equipment.
Can I use this calculator for water, oil or other liquids?
Yes. The formula \( q = h \, A \, \Delta T \) applies to any fluid — gases, water, oil, refrigerants. The only thing that changes is the heat-transfer coefficient: liquids generally transfer heat far better than gases, so \( h \) for flowing water can be hundreds of times larger than for air. Just pick an \( h \) value appropriate to your liquid and flow speed.
What is the difference between heat-transfer rate and heat flux?
Heat-transfer rate \( q \) is the total energy crossing the whole surface per second, measured in watts. Heat flux \( q/A \) is the same energy per unit of surface area, in W/m². A large radiator and a small chip can have identical flux but very different total heat rates. Flux is useful for comparing surfaces or checking whether a component exceeds a safe thermal density.
What are common mistakes when using q = h·A·ΔT?
The most frequent errors: using absolute temperatures (e.g. 60 °C) instead of the difference between surface and fluid; using an \( h \) value from a different situation (free-air value for a fan-cooled surface); forgetting that both sides of the surface matter (only the area in contact with the fluid counts); and ignoring radiation, which in still air or at high temperature can carry a comparable amount of heat and must be added separately.
Does this formula include heat lost by radiation or conduction?
No. \( q = h \, A \, \Delta T \) covers convection only. Real surfaces simultaneously radiate to surroundings (roughly \( q_{rad} = \varepsilon \sigma A (T_s^4 - T_{sur}^4) \)) and exchange heat by conduction through solid parts. At moderate temperatures with moving air, convection usually dominates; near room temperature in still air, radiation can be of the same order. For total heat loss, compute each mode separately and add them.