R-Value to U-Factor Calculator (R-Value, RSI & U-Factor Converter)
Enter an R-value, RSI or U-factor and get all four forms at once, or pick an insulation material and a thickness to see what that layer works out to. Everything runs in your browser.
Quick chart: common R-values
| R-value (IP) | RSI m²·K/W | U-factor (IP) BTU/h·ft²·°F | U-value (SI) W/m²·K |
|---|---|---|---|
| R-13 | 2.29 | 0.0769 | 0.437 |
| R-19 | 3.35 | 0.0526 | 0.299 |
| R-30 | 5.28 | 0.0333 | 0.189 |
| R-38 | 6.69 | 0.0263 | 0.149 |
Unit conversions only, rounded for reading; not product ratings or code minimums. Longer charts are in the guide's reference tables. To add up several layers in series, use the R-value converter with layer totals.
Formulas and sources
U = 1 ÷ R (in matching units)
RSI = R(imperial) × 0.1761101838
U(W/m²·K) = U(BTU/h·ft²·°F) × 5.678263
R(layer) = thickness (in) × R per inch
- The factor 5.678263 W/m²·K per BTU/h·ft²·°F is the standard SI/IP conversion listed in the ASHRAE Handbook—Fundamentals.
- Open-cell (R-3.6 per inch) and closed-cell (R-6.1 per inch) spray foam values are those quoted from the U.S. DOE Building America guide in the guide's comparison table. The fiberglass figure is a 6 in R-19 batt divided by 6.
- The cellulose and XPS values are illustrative starting points, not product ratings. Real products vary by density, age and temperature; use the manufacturer's label.
Frequently asked questions
How do I convert R-value to U-factor?
Take the reciprocal: U = 1 ÷ R in matching units. An R-13 wall has an imperial U-factor of 1 ÷ 13 ≈ 0.0769 BTU/h·ft²·°F. For a metric U-value in W/m²·K, divide 5.678 by the imperial R-value: 5.678 ÷ 13 ≈ 0.437.
How do I convert R-value to RSI?
Multiply the imperial R-value by 0.176110 (or divide by 5.678263). R-19 is about RSI 3.35 m²·K/W.
How do I find the R-value of an insulation layer from its thickness?
Multiply the thickness in inches by the material's R-value per inch. For example, 3.5 in of open-cell spray foam at R-3.6 per inch gives about R-12.6. Use the labelled value for the product you are considering.
Are U-factor and U-value the same?
They describe the same property, thermal transmittance. In North America U-factor usually means imperial units (BTU/h·ft²·°F), while U-value usually means metric (W/m²·K). 1 BTU/h·ft²·°F equals 5.678 W/m²·K.
Does this calculator tell me if my wall meets code?
No. It converts numbers you enter. It does not measure any product and does not check an assembly against an energy code; consult your local code authority for requirements.
Limits
- A single layer's R-value ignores framing and thermal bridges, so it overstates a whole wall. See the guide's thermal bridging example.
- Window U-factors from rating labels describe the whole product; do not add them to wall layers.
- Results are calculations from the numbers you enter, not measurements.
Background and worked examples: U-value and R-value: how to convert U-factor to R-value (and RSI).