Measure or otherwise ascertain the thickness, in inches, of the material for which you have an R-value. For example, a roll of insulation may have the thickness of 3.5 inches, specified on the packaging. Record this value.

Divide the thickness by the R-value. The result is the K-value. For example: Blown fiberglass has a typical R value of 2.20 per inch thickness. The K-value of a 4-inch thick layer is 1.82 because 4 / 2.2 = 1.8181.

Reverse your calculations to check for errors. Multiply the K-value by the R-value. The result should be the thickness of the material. If your answer differs from the thickness value, there was an error in your calculations. Repeat the math until the result is correct.

## Tip

- The K-value of an insulating material will be clearly marked on the packaging or technical data sheet accompanying the product.