heat transfer formulas and interpretation
Heat moves through solids by conduction, between surfaces and fluids by convection, and through electromagnetic radiation.
The calculator preserves all three modes and metric or imperial engineering inputs.
How to use the heat transfer calculator
- Choose a model: Select the relationship matching the problem.
- Choose the unknown: Select the quantity to calculate.
- Enter values: Enter all known values with matching units and signs.
- Calculate: Review the result, formula, units, and direction.
Formula and variables
Each mode uses its corresponding steady heat-transfer relationship.
Q̇cond = kAΔT/L; Q̇conv = hAΔT; Q̇rad = εσA(T₁⁴−T₂⁴)- Q̇ — Heat-transfer rate
- Thermal energy per time (W)
- k — Conductivity
- Material thermal conductivity (W/(m·K))
- h — Convection coefficient
- Surface heat-transfer coefficient (W/(m²·K))
- ε — Emissivity
- Surface radiation effectiveness (dimensionless)
- A — Area
- Transfer area (m²)
Wall conduction example
A wall has k = 0.5 W/(m·K), area 10 m², thickness 0.2 m, and ΔT = 20 K.
- Conductivity
- 0.5 W/(m·K)
- Area
- 10 m²
- Thickness
- 0.2 m
- Q̇ = kAΔT/L
- Q̇ = 500 W
Result: Conduction heat rate is 500 W.
The result assumes one-dimensional steady conduction.
Understanding your results
Interpreting the result
Real systems may combine modes and include contact resistance, geometry factors, and changing properties.
Assumptions
- The selected equation represents the physical system.
- Inputs use a consistent reference direction.
- Values are converted through coherent SI units.
Limitations
- Vector components must be resolved along a common axis.
- External forces or energy losses are not added automatically.
- Results depend on the accuracy of entered measurements.
Common mistakes
- Mixing incompatible units.
- Dropping negative signs that represent direction.
- Using weight where mass is required.
- Entering a zero divisor.
Practical use cases
Physics problems
Check classroom, laboratory, and mechanics calculations.
Practical estimates
Estimate motion, forces, and energy for real systems.
Frequently asked questions
Can a result be negative?
Yes. For directional quantities, the sign indicates direction relative to the chosen positive axis.
Should I use SI units?
The interface can convert supported units, while the formulas are evaluated through coherent SI units.
Sources and review
- SI Brochure, 9th edition — BIPM. Accessed 2026-07-11.
- Special Publication 811 — NIST. Accessed 2026-07-11.
Reviewed 2026-07-11.