Heat Transfer Calculator
Calculate conductive heat flow through a wall, and its R and U values.
Please note: These are idealised textbook models using representative material data. They are for study, sizing and sanity-checking only — never for final design. Real engineering requires code-compliant analysis, verified material certificates and a qualified engineer’s sign-off.
What the Heat Transfer Calculator does
Conduction moves heat through a solid in proportion to its conductivity, area and temperature difference, and inversely with thickness. R-value inverts that into a resistance so layers can simply be added, and U-value inverts it back for comparison against building standards.
Formula
Q = k A ΔT ÷ L (Fourier’s law)R = L ÷ kU = 1 ÷ RLayered walls: R_total = R₁ + R₂ + R₃ …
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Material | one of 13 options | Yes | — |
| Custom thermal conductivity | W/m·K | Optional | Overrides the material if set. Mineral wool is about 0.04, EPS 0.035. Accepts 0 or more. |
| Surface area | number | Yes | — |
| Surface area unit | one of 5 options | Yes | — |
| Thickness | number | Yes | — |
| Thickness unit | one of 9 options | Yes | — |
| Temperature difference | number | Yes | — |
| Temperature difference unit | one of 3 options | Yes | — |
| Hours of operation | number | Optional | Accepts 0 or more. |
How to use it
- Choose Material and Surface area unit.
- Enter Surface area, Thickness and Temperature difference.
- Optionally add Custom thermal conductivity and Hours of operation.
- Select Calculate.
Worked example
A 10 m² concrete wall 200 mm thick with 20 °C across it.
- Material
- Concrete
- Area
- 10 m²
- Thickness
- 200 mm
- ΔT
- 20
Q = 1.7 × 10 × 20 ÷ 0.2 = 1,700 W. R = 0.118 m²·K/W and U = 8.5 W/m²·K — very poor by modern standards.
Frequently asked questions
What is the difference between R-value and U-value?
They are reciprocals. R measures resistance and adds up across layers; U measures transmittance and is what building codes specify. Low U and high R both mean good insulation.
Why do US and metric R-values differ?
Different units. A US R-value in ft²·°F·h/BTU is about 5.68 times the SI value in m²·K/W for the same material, so R-19 US is roughly 3.3 SI.
Method and sources
Method. Conduction by Fourier's law, Q = kAΔT ÷ L, with convection and radiation modes available, using representative material conductivities.
Assumptions
- Steady state, one-dimensional flow, and constant conductivity across the temperature range.
Limitations
- Steady state excludes transient behaviour entirely, which is often the question that matters — how long something takes to warm up, not its final rate.
- Real assemblies have contact resistance at every interface and thermal bridges through fixings, both of which can dominate the calculated path.
- Conductivity values are representative for a material class, not certified figures for a specific product; a manufacturer's declared value governs.
Sources
- Fourier's law of heat conduction, with material data from product declarations — Classical heat transfer; conductivity values vary by product. The conduction relationship. Declared conductivity for a specific insulation or material comes from its datasheet.