Heat Transfer Calculator
The rate of heat conduction through a wall, window or insulation layer — with the R-value, U-value and running cost.
How fast heat leaks through a wall.
How the heat transfer calculator works
Fourier's law: Q̇ = kA ΔT ÷ d. Heat flows in proportion to area and temperature difference, and inversely with thickness. Thermal conductivity k is the material property; its reciprocal, thickness over k, is the R-value builders use.
The two are worth keeping straight. R-value adds up across layers, U-value is its reciprocal, and it is U that appears in building codes.
Formula: Q̇ = k A ΔT / d; R = d/k; U = 1/R
Worked examples
| Inputs | Heat flow | Note |
|---|---|---|
| 110 mm brick, 20 m², 20 K | 2,618.18 W | 2,618 W — a very poor wall |
| The same wall with 100 mm batts | 160 W | 160 W — sixteen times better |
| A single-glazed window | 10,000 W | the glass itself barely resists at all |
FAQFrequently asked questions
What is the difference between R-value and U-value?
R-value measures resistance and adds across layers; U-value is its reciprocal and measures how much heat gets through. Lower U is better.
Does this include air films and cavities?
No — it is the conduction through one solid layer. A real wall assembly adds surface films and any air gaps, which raize the total R.
Why is insulation so effective?
Because it traps still air, whose conductivity is about 0.026 W/m·K. Fibreglass and foam are mostly air held in place.
Why do single-glazed windows lose so much heat?
Not because glass conducts well, but because 4 mm of anything resists almost nothing. Double glazing works by adding a trapped air or argon gap.
Is a steel stud a problem?
Yes — it is a thermal bridge. Steel conducts about 1,000 times better than insulation, so a stud can undo much of a wall's rating.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.