Insulation Thickness (R-Value) Calculator

How it is calculated

The tool divides the R-value you want by the R-value the material gives per inch: thickness (in) = Rtarget ÷ Rper inch. It then converts to centimetres with cm = in × 2.54. Both numbers must be greater than zero, otherwise there is no result. It shows both lengths to two decimals. The calculation assumes one uniform layer of a single material, because R-values of layers in series simply add up. Values are US-style R-values (per inch of thickness); a product's rated R-per-inch is on its label, and it can change if the material is compressed or has gaps. Local code minimums differ by country and climate, so take the target from your own rules.

Common mistakes

• Entering the total R-value of a product (for example R-19 on a batt) in the per-inch field; that field wants R per inch, and the total is the target.
• Mixing metric RSI values with US R-values; the formula works only if the target and the per-inch value use the same system.
• Forgetting existing insulation: if the attic already has R-11, subtract it from the target before dividing, or the answer will be too thick.
• Assuming compressed material keeps its rating; squeezed batts lose R per inch, so do not cram more material into a thinner space.
• Rounding the thickness down; round up to the next available product thickness so you do not fall below the target.