Convection Heat Transfer Calculator
Calculate convection heat transfer rate, heat flux, convective thermal resistance, required heat-transfer coefficient, required surface area, and surface or fluid temperature using Newton's law of cooling. It is intended for electronics thermal screening when h is known or reasonably estimated from separate data.
Engineering tool
Convection Heat Transfer Calculator
Calculate steady-state convection heat transfer with a user-provided heat-transfer coefficient, effective area, and surface/fluid temperature difference.
Enter a known or assumed convection coefficient; this calculator does not estimate h from flow conditions.
Use exposed convection area, not automatically the PCB footprint or package top area.
Result console
- Heat Transfer Rate
- 300W
- Heat Flow Direction
- Surface -> Fluid
- Temperature Difference
- 60K
- Heat Flux
- 1500W/m²
- Convective Resistance
- 0.2K/W
- Convective Conductance
- 5W/K
Steady-state convection only. The heat-transfer coefficient must be known or separately estimated; radiation, fin efficiency, solid conduction, and transient cooling are outside this calculator.
Formula
Formula reference
Newton convection heat-transfer formulas
The calculator keeps heat-transfer magnitude non-negative and reports whether heat flows from surface to fluid or from fluid to surface.
Qdot = hA(Ts - T∞)|Qdot| = hA|Ts - T∞|q'' = Qdot/A = h(Ts - T∞)Rθ,conv = 1/(hA)Gconv = hAhrequired = Qdot/(A|ΔT|)Arequired = Qdot/(h|ΔT|)Surface-to-fluid: Ts = T∞ + Qdot/(hA)Surface-to-fluid: T∞ = Ts - Qdot/(hA)Variable definitions
- Qdot
- convection heat-transfer rate, W
- q''
- convection heat flux, W/m²
- h
- heat-transfer coefficient, W/(m²·K)
- A
- effective exposed surface area, m²
- Ts
- surface temperature
- T∞
- bulk or free-stream fluid temperature
Convection Heat Transfer Formula Audit
| Newton Convection Formula | Qdot = hA(Ts - T∞). |
|---|---|
| Heat Flow Sign Convention | Positive signed Q means surface-to-fluid for the chosen Ts - T∞ convention. |
| Heat Transfer Magnitude Formula | |Qdot| = hA|Ts - T∞|. |
| Heat Flux Formula | q'' = Qdot/A = h(Ts - T∞), with magnitude h|ΔT|. |
| Convective Resistance Formula | Rθ,conv = 1/(hA), not 1/h. |
| Convective Conductance Formula | Gconv = hA. |
| Required h Formula | hrequired = Qdot/(A|ΔT|). |
| Required Area Formula | Arequired = Qdot/(h|ΔT|). |
| Surface Temperature Formula | Surface-to-fluid: Ts = T∞ + Qdot/(hA); fluid-to-surface: Ts = T∞ - Qdot/(hA). |
| Fluid Temperature Formula | Surface-to-fluid: T∞ = Ts - Qdot/(hA); fluid-to-surface: T∞ = Ts + Qdot/(hA). |
| h Units | W/(m²·K), numerically equal to W/(m²·°C) for intervals. |
| Area Units | m², cm², mm² and in² convert internally to m². |
| Heat Rate Units | W, mW and kW convert internally to W. |
| Heat Flux Units | W/m² primary; W/cm² formatting is supported by shared utilities. |
| Thermal Resistance Units | K/W and °C/W are numerically equivalent for this model. |
| Absolute Temperature Convention | °C, K and °F are converted as absolute temperatures and cannot be below 0 K. |
| Temperature Difference Convention | Δ1°C = 1K = 1.8°F difference. |
| Surface / Fluid Direction Convention | Surface temperature may be above or below fluid temperature; direction is reported separately. |
| Effective Area Convention | Use exposed effective convection area; fin efficiency is not derived. |
| Thermal Conductivity vs h Boundary | k is W/(m·K) for conduction; h is W/(m²·K) for a convection boundary. |
| Natural Convection Boundary | Natural-convection h may be entered if known, but correlations are not solved. |
| Forced Convection Boundary | Forced-convection h may be entered if known, but fan/flow correlations are not solved. |
| Typical h Guidance Policy | Any h ranges are screening guidance only, not automatic calculation. |
| Fin Efficiency Boundary | No fin efficiency, bypass, or extended-surface model. |
| Radiation Boundary | No Stefan-Boltzmann radiation or total heat-loss model. |
| Conduction Boundary | No internal solid conduction; use THM-008 or a thermal network. |
| Transient Boundary | No temperature-vs-time response; use THM-009. |
| THM-002 Scope Separation | Heat-sink sizing and RθSA budgeting remain in THM-002. |
| THM-008 Scope Separation | Solid conduction and k-based geometry remain in THM-008. |
| THM-009 Scope Separation | Thermal RC transient response remains in THM-009. |
| THM-011 Scope Separation | Electronics enclosure temperature-rise design remains planned for THM-011. |
Worked Examples
| Case | Calculation | Result |
|---|---|---|
| Heat-transfer rate | h=25, A=.2m², Ts=80°C, T∞=20°C | ΔT=60K, Q=300W, surface -> fluid |
| Heat flux | 25×60 | q''=1500W/m² |
| Convective resistance | 1/(25×.2) | Rθ,conv=.2K/W |
| Convective conductance | 25×.2 | Gconv=5W/K |
| Required h | 300/(.2×60) | h=25W/(m²·K) |
| Required area | 300/(25×60) | A=.2m² |
| Solve surface temperature | T∞=20°C, Q=300W, hA=5W/K | Ts=80°C |
| Solve fluid temperature | Ts=80°C, Q=300W, hA=5W/K | T∞=20°C |
| Zero temperature difference | Ts=T∞ | Q=0 |
| Reverse heat flow | Ts=20°C, T∞=80°C | |Q|=300W, fluid -> surface |
| Double h | h 25 -> 50 | Q doubles |
| Double area | A .2 -> .4 | Q doubles |
| Double ΔT | 60K -> 120K | Q doubles |
| Double h resistance | h 25 -> 50 | Rθ halves |
| Double area resistance | A .2 -> .4 | Rθ halves |
| Resistance identity | hA×Rθ | 1 |
| Heat flow identity | Q×Rθ | |ΔT| |
| Flux identity | q''×A | Q |
| Option A | h=50, A=.05 | G=2.5W/K |
| Option B | h=20, A=.2 | G=4W/K; better despite lower h |
| Area conversion | 1cm² | 1e-4m² |
| Area conversion | 100cm² | .01m² |
| Area conversion | 1mm² | 1e-6m² |
| Area conversion | 1in² | .00064516m² |
| Resistance units | 1K/W | 1°C/W numerically |
| Temperature difference | Δ10°C | 10K |
| Fahrenheit difference | Δ10°C | 18°F difference |
| Absolute temperature | 77°F | 25°C |
| Zero h | h=0 | rejected |
| Zero area | A=0 | rejected |
| Negative h | h<0 | rejected |
| Negative area | A<0 | rejected |
| Required h zero ΔT | Q>0 and ΔT=0 | no finite solution |
| Invalid number | NaN | rejected |
| Invalid number | Infinity | rejected |
Engineering Notes
- Convective heat transfer follows Q=hA(Ts-T∞) for the stated steady boundary model.
- The heat-transfer coefficient h is not a universal material constant.
- h depends on fluid, velocity, geometry, orientation, surface condition, flow regime and temperature.
- Heat flows from the higher-temperature side toward the lower-temperature side.
- T∞ represents bulk or free-stream fluid temperature, not always room ambient.
- Convective thermal resistance is 1/(hA), so effective area matters as much as h.
- Compare designs by hA conductance, not by h alone.
- Natural and forced convection can have very different h values.
- Typical h ranges are rough guidance only and should not replace measured or vendor data.
- Use effective exposed area; fins require an area already adjusted for efficiency.
- Radiation, solid conduction, and thermal transients are outside this calculator.
- Critical electronics cooling should be verified with measurement, vendor models, or simulation.
Common Mistakes
- Treating h as thermal conductivity k.
- Treating k as a convection coefficient.
- Using 1/h as total convection resistance instead of 1/(hA).
- Forgetting to multiply by surface area.
- Mixing heat-transfer rate W with heat flux W/m².
- Using package footprint as the full convection area.
- Ignoring fluid or free-stream temperature.
- Assuming the surface is always hotter than the fluid.
- Using absolute Fahrenheit conversion for a temperature difference.
- Entering cm² as m².
- Treating natural-convection h as a fixed constant.
- Treating forced-convection h as a fixed constant.
- Using hA as a complete heat-sink model.
- Ignoring fin efficiency when using extended surfaces.
- Ignoring radiation while claiming total heat loss.
- Treating the steady convection relation as a full transient cooling model.
Support reference
FAQ
What is convection heat transfer?
Convection heat transfer is heat exchange between a surface and a moving or stationary fluid next to that surface.
What is Newton's law of cooling?
In this calculator, Newton's law of cooling means the steady convection boundary relation Qdot = hA(Ts - T∞). It is not the full exponential transient cooling model.
How do I calculate convection heat transfer?
Use Qdot = hA(Ts - T∞). The calculator reports heat-transfer magnitude as a non-negative value and separately shows the heat-flow direction.
What is the heat transfer coefficient h?
h is the convective heat-transfer coefficient. It relates heat flux to the temperature difference between a surface and the bulk or free-stream fluid.
What units are used for h?
The SI unit for h is W/(m²·K). W/(m²·°C) is numerically equivalent for temperature differences.
What is the difference between thermal conductivity k and convection coefficient h?
Thermal conductivity k is a material conduction property with units W/(m·K). The convection coefficient h is a boundary coefficient with units W/(m²·K).
How do I calculate convection heat flux?
Use q'' = Qdot/A = h(Ts - T∞). Heat flux is measured in W/m², while total heat-transfer rate is measured in watts.
What is convective thermal resistance?
Convective thermal resistance is Rθ,conv = 1/(hA). It depends on both the heat-transfer coefficient and the effective surface area.
Why is convection thermal resistance 1/(hA)?
Because Qdot = hAΔT can be written as Qdot = ΔT/Rθ, so Rθ,conv = 1/(hA).
What is convective conductance?
Convective conductance is hA in W/K. It is the total convection capability for the entered surface and fluid condition.
How do I calculate the required heat transfer coefficient?
Use hrequired = Qdot/(A|ΔT|). Positive heat transfer at zero temperature difference has no finite h solution.
How do I calculate required surface area?
Use Arequired = Qdot/(h|ΔT|). The area must be effective exposed convection area, not simply footprint area.
How do I calculate surface temperature from convection?
For surface-to-fluid heat flow, Ts = T∞ + Qdot/(hA). For fluid-to-surface heat flow, Ts = T∞ - Qdot/(hA).
What is free-stream temperature?
Free-stream temperature is the fluid temperature away from the immediate thermal boundary layer near the surface.
What is bulk fluid temperature?
Bulk fluid temperature is the representative fluid temperature used with the convection coefficient. In many electronics examples it is close to local air temperature, but it should match the chosen h data.
What is natural convection?
Natural or free convection is fluid motion driven mainly by buoyancy from temperature differences, not by a fan or pump.
What is forced convection?
Forced convection uses externally driven fluid motion such as a fan, blower, or pump, and can have much higher h than natural convection.
What affects the convection coefficient?
h depends on fluid, velocity, geometry, orientation, surface condition, flow regime, temperature, and characteristic length.
Can I use a typical h value for air?
Typical h values are rough screening inputs only. Critical designs should use measured data, vendor thermal data, correlations, CFD, or testing.
Does this calculator include radiation?
No. It calculates Qconv only. Real surface heat loss may also include radiation.
Does this calculator calculate fin efficiency?
No. Enter effective area only if fin efficiency or overall surface efficiency is already known.
What is the difference between THM-008 conduction and THM-010 convection?
THM-008 calculates heat flow through solids using thermal conductivity k. THM-010 calculates a surface-fluid convection boundary using h.
What is the difference between THM-009 cooling and THM-010 convection?
THM-009 estimates first-order temperature versus time. THM-010 is a steady convection relation at known temperatures or known heat rate.
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