Thermal insulation
R-value to U-value calculator
Convert R-value to U-value or U-factor and back. Compare US and SI units, RSI, formulas and worked examples.
How to use this calculator
Method and boundary
Select conversion direction and the R/U unit system first. The entered number must use the displayed input unit; thermal-unit mode is independent of length preference.
Within the same unit system: U = 1/R and R = 1/U
Example
R = 2.5 m²·K/W corresponds to U = 0.4 W/(m²·K) when both describe the same scope.
Unit conversion or taking a reciprocal does not add omitted layers or surface resistances. Insulation R does not thereby become whole-wall U.
Calculation privacy
Recent stores calculator ID, route ID, locale, time and unit system only.
How do you convert R-value to U-value?
R-value measures thermal resistance; U-value measures thermal transmittance. They are reciprocals for the same material or assembly scope in matching units. A US insulation label does not give the same numerical R-value as SI units.
- SI R-value (RSI): U [W/(m²·K)] = 1 / R [m²·K/W].
- US R-value: U-factor [Btu/(h·ft²·°F)] = 1 / R [h·ft²·°F/Btu].
- US R-value to SI: RSI ≈ R(US) / 5.678263337; U(SI) ≈ 5.678263337 / R(US).
R-13, R-19 and R-30 conversion table
Our own numerical conversion examples. This table does not specify required insulation or the U-value of a complete wall; each R-value must cover the same scope as the U-value being compared.
| R (US) h·ft²·°F/Btu | RSI m²·K/W | U (SI) W/(m²·K) | U-factor (US) Btu/(h·ft²·°F) |
|---|---|---|---|
| R-13 | 2.2894 | 0.4368 | 0.0769 |
| R-19 | 3.3461 | 0.2989 | 0.0526 |
| R-30 | 5.2833 | 0.1893 | 0.0333 |
| R-38 | 6.6922 | 0.1494 | 0.0263 |
| R-49 | 8.6294 | 0.1159 | 0.0204 |
R-value and U-value conversion questions
Does R-30 mean a U-value of 0.033?
US R-30 gives a U-factor of about 0.03333 Btu/(h·ft²·°F), but an SI U-value of about 0.18928 W/(m²·K). A value of 0.033 without a unit can therefore give a misleading comparison.
What is the difference between R-value and RSI?
RSI is thermal resistance in m²·K/W. US R-value is usually expressed in h·ft²·°F/Btu. US R-13 is approximately RSI 2.289. Canadian labels can use either notation, so check the unit on your document.
Are U-value and U-factor the same thing?
Both describe thermal transmittance, but their units and tested scope must match. Do not compare the U-factor of a complete window with the R-value of an insulation layer. Select SI or US according to the document.
Can I calculate a whole wall U-value from insulation R-value?
Not by taking the reciprocal of one insulation board's R-value. Total resistance must cover the relevant layers and surface resistances; thermal bridges and non-uniform parts may need a different method. This tool converts a number and does not design an assembly.