Convection Calculator

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Heat transfer coefficient (h)

W/(m²·K)

Exchange area (A)

m²

Temperature difference (ΔT)

K / °C

Decimal places

Complete the form and click the button to see the result.

What is thermal convection?

Convection (heat transfer) is the process of heat transfer between the surface of a solid and the surrounding moving fluid (liquid or gas). The calculations are based on the so-called Newton's cooling law.

Mathematical formula describing this process:

            Q = h * A * ΔT
        

Q – heat flux (heat power transferred by convection) expressed in Watts [W].
h – heat transfer coefficient [W/(m²·K)], determining the intensity of heat transfer at the phase boundary.
A – heat transfer area [m²] where convection occurs.
ΔT – temperature difference between the surface and the fluid [K or °C].

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Thermal convection calculator - Newton's law in practice

The tool calculates heat transfer by convection based on the equationQ̇ = h · A · (T_s- T_∞). You select the calculation mode and indicate the unknown:Qdot, h, AorΔT. The calculator also allows you to estimatehfrom dimensionless correlations for natural and forced convection in typical geometries, and then calculates the full power balance for a given surface and temperature difference.

thermodynamics heat exchange convection HVAC process engineering

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Model and patterns

Newton's law of cooling

Q̇ = h · A · (T_s - T_∞)

Q̇- thermal power transferred by convection [W]
h- heat transfer coefficient [W/(m² K)]
A- heat exchange area [m²]
T_s- surface temperature [°C or K]
T_∞- ambient or fluid temperature away from the surface [°C or K]

Forced convection - estimation of h

We use correlations with dimensionless numbersRe, Pr, Nu. For parallel flow over a plate of lengthLin the direction of flow:

Nu_L = 0.664 · Re_L^{1/2} · Pr^{1/3} - lamininar layer
Nu_L = 0.037 · Re_L^{0.8} · Pr / [1 + 2.443 · Re_L^{-0.1} · (Pr^{2/3} - 1)] - generalized

h = Nu_L · k / L

Wherekis the thermal conductivity of the fluid [W/(m·K)],Re = ρ·v·L/µ, Pr = c_p·μ/k.

Natural convection - estimation of h

For vertical height boardLin the air in the typical range of numbersRa = Gr·Pr:

Nu_L = 0.68 + 0.670 · Ra_L^{1/4} / [1 + (0.492/Pr)^{9/16}]^{4/9}
h = Nu_L · k / L

We assume constant fluid properties at the film temperatureT_f = (T_s + T_∞)/2.

Transformations into calculator modes

Q̇ mode: Q̇ = h · A · ΔT

h mode: h = Q̇ / (A · ΔT)

A mode: A = Q̇ / (h ΔT)

ΔT mode: ΔT = Q̇ / (h A)

Assumptions and limitations

Steady state and small temperature difference for constant fluid properties.
No radiation - if important, add the radiation component separately.
Uniform surface temperature field and uniform A.

Form fields and supported units

mode- selection of unknown: Qdot, h, A, dT.
Qdot- thermal power [W, kW].
h- heat transfer coefficient [W/(m²·K)].
A- area [m², cm²].
T_s, T_∞- temperature [°C or K].
dT- temperature difference [K].
Preset h- quick selection of the h range for a typical case.
precision- number of decimal places for formatting the result.

Size	Symbol	Units	Default
Convective power	Q̇	W, kW	W
Capture rate	h	W/(m² K)	W/(m² K)
Surface	A	m², cm²	m²
Temperature difference	ΔT	K, °C	K

Guide values h - quick selection

Case	Description	h [W/(m² K)]
Air - natural convection	Vertical plate, calm air	2 - 10
Air - forced convection	Blowing 1 - 5 m/s	20 - 100
Water - natural convection	Calm water, small ΔT	50 - 500
Water - forced convection	Flow in the pipe or over the plate	500 - 10,000
Fin exchanger	Fins in the air stream	50 - 300
Turbulent cooling	High water velocity	10,000 - 30,000

Calculation examples

Example 1 - calculate Q̇ from given h

h = 35 W/(m²·K)
A = 1.2 m²
T_s= 55°C, T_∞= 25°C

ΔT = 30 K. Q̇ = 35 · 1.2 · 30 = 1260 W.

Example 2 - calculate h from the power measurement

Q̇ = 480 W
A = 0.8 m²
ΔT = 20 K

h = 480 / (0.8 · 20) = 30 W/(m² · K).

Example 3 - required A for a given power

Q̇ = 1500 W
h = 25 W/(m²·K)
ΔT = 20 K

A = 1500 / (25 · 20) = 3.0 m². Increasing the surface area with fins will lower the temperature.

Example 4 - ΔT for heat sink

Q̇ = 90 W
h = 12 W/(m² K)
A = 0.25 m²

ΔT = 90 / (12 0.25) = 30 K. At T_∞= 22°C the heat sink will reach approximately 52°C.

Inputs for estimating h from correlation

k- thermal conductivity of the fluid [W/(m·K)]
μ- dynamic viscosity [Pa·s]
ρ- density [kg/m³]
c_p- specific heat [J/(kg·K)]
v- flow velocity [m/s] for forced convection
L- characteristic length [m]

The calculator calculates dimensionless numbers and, based on them, selectsNuandh. Properties as a function of film temperature are available for water and air.

Common pitfalls

Do not mix °C with K. The temperature difference in °C is equal to the value in K.
Make sure that A refers to the actual exchange area. Heat sinks have a larger effective A thanks to the fins.
High flow rate increases h, but the pressure drop and pumping cost also increase.
Radiation can be comparable to convection at high temperatures. Add the radiation component separately.
The properties of a fluid depend on temperature. Use film temperature T_f.

Practical applications

Design of electronics heat sinks and heat exchangers.
Selection of surfaces for HVAC radiators and coolers.
Estimation of heat losses of tanks, pipes and housings.
Assessment of exchange intensification by blowing and turbulization.
Verification of CFD results with quick engineering calculations.

FAQ

What is the typical h value for unsupplied air

Typically 2 - 10 W/(m²·K) for a vertical slab in still air.

Can I add convection and radiation

Yes. Treat the total power as Q̇ = Q̇_conv+ Q̇_rad. Both mechanisms work in parallel.

Are the correlations accurate for each case

No. They give an estimate of several percent. Testing or CFD is required for non-standard geometries or complex flows.

How to improve convection cooling

Increase A, increase the flow rate, use fins, lower the ambient temperature, uniform the surface temperature distribution.

Does the calculator take into account the variation of h along the surface

No. Uses an average value. For accurate analyses, use local correlations or CFD.

Summary

The convection calculator simplifies the design and analysis of heat transfer between a surface and a flowing fluid. You can calculate the power, required exchange field, temperature difference or transfer coefficient in one place. Additional correlations allow h to be estimated in air and water for common geometries, which accelerates design decisions and proof of concept. Combined with the radiation and conductivity calculator you will get a complete picture of complex heat losses and gains.