What this calculator does
Thermal resistance describes how strongly a layer of material resists heat flowing through it: a higher R-value means better insulation. It depends on three things: how thick the layer is, how conductive the material itself is (its k-value), and the area the heat is passing through.
This calculator finds the thermal resistance from those three inputs, along with the U-value, which is simply its reciprocal and is the figure more commonly quoted for windows, walls and insulation products in building specifications.
The formula
Divide the thickness by the product of thermal conductivity and area: R = thickness ÷ (conductivity × area). The U-value is 1 ÷ R.
| Term | Meaning |
|---|---|
| Thermal conductivity (k) | How readily a specific material conducts heat, in watts per metre-kelvin (W/(m·K)). Lower values mean a better insulator; common insulation batts sit around 0.03–0.04 W/(m·K). |
| R-value | Thermal resistance: thickness divided by (conductivity × area), in kelvin per watt (K/W). Higher is more resistant to heat flow. |
| U-value | The reciprocal of R-value, in watts per kelvin (W/K), commonly used to rate windows and building elements, where lower U-values mean better insulation. |
The inputs explained
| Field | What to enter |
|---|---|
| Thickness (m) | The thickness of the material layer, in metres. |
| Thermal conductivity (k) (W/(m·K)) | The thermal conductivity of the specific material, in W/(m·K). Check the product's technical data sheet for an accurate figure. |
| Area (m²) | The area the heat is passing through, in square metres. |
When to use it
Comparing insulation thicknesses
Running the same material and area at different thickness values shows directly how much extra resistance a thicker layer adds.
Comparing two materials
Swapping the conductivity value for a different material at the same thickness and area shows which one resists heat flow better.
Checking a manufacturer's R-value claim
Recalculating from the thickness and conductivity a manufacturer states is a way to independently check a quoted R-value.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does thermal resistance change with insulation thickness, at k = 0.04 W/(m·K) over 10 m²?
A common insulation conductivity value, with thickness varied.
| Thickness | Thermal resistance (R) | U-value (1/R) |
|---|---|---|
| 0.05 m | 0.1250 K/W | 8.000 W/K |
| 0.1 m | 0.2500 K/W | 4.000 W/K |
| 0.15 m | 0.3750 K/W | 2.667 W/K |
| 0.2 m | 0.5000 K/W | 2.000 W/K |
| 0.3 m | 0.7500 K/W | 1.333 W/K |
Questions
Why does area matter for thermal resistance?
A larger area gives heat more total pathway to flow through in parallel, which lowers the overall resistance of that layer: the same material and thickness resists heat less over a bigger area.
Is a higher or lower R-value better for insulation?
Higher. A higher R-value means the layer resists heat flow more strongly, which is what's wanted for insulation.
How is this different from the R-values on insulation batt packaging?
The same idea, though packaged batt R-values are usually already calculated per square metre of typical installation and may include added standard air gaps; this calculator computes R directly from thickness, conductivity and the actual area entered.
What conductivity value should I use?
The specific material's technical data sheet gives the most accurate figure. Typical insulation batts sit around 0.03–0.04 W/(m·K); denser materials like concrete or brick have much higher (worse) conductivity, often above 1 W/(m·K).
For heat transfer over time rather than through a static layer, see a related physics calculator, or for building material quantities, see the timber beam span calculator.