Thermal insulation
Insulation thickness calculator
Calculate insulation thickness in mm and inches from your target RSI or US R-value and product lambda, k-value or R per inch.
How to use this calculator
Method and boundary
Choose target R units: SI m²·K/W or US h·ft²·°F/Btu. Enter material data as lambda, k or US R per inch. Changing units converts a positive value. Record the source of this layer's target R.
Thickness [m] = target layer R × design λ
Example
Target R = 3 m²·K/W with hypothetical λ = 0.040 gives 120 mm, about 4.724 in. US R = 13 with hypothetical R-4 per inch gives 3.25 in, or 82.55 mm.
This is nominal single-layer thickness. The calculator neither establishes target R nor selects stock thickness or assesses the complete assembly.
Calculation privacy
Recent stores calculator ID, route ID, locale, time and unit system only.
How to calculate insulation thickness from R-value
For one homogeneous layer in SI, thickness in metres is d = R × λ. With target US R and k in Btu·in/(h·ft²·°F), thickness in inches is d = US R × k. With R per inch, use d [in] = target US R / R per inch. All data must describe the same material and conditions.
SI example: R = 3 and λ = 0.040
Target R = 3 m²·K/W with hypothetical λ = 0.040 W/(m·K) gives 3 × 0.040 = 0.12 m, or 120 mm. That is about 4.724 in. The result does not select an available stock size or establish the building's required R.
| Target US R | Thickness · in | Thickness · mm |
|---|---|---|
| R-13 | 3.25 | 82.55 |
| R-19 | 4.75 | 120.65 |
| R-30 | 7.5 | 190.5 |
Where does the target R come from?
From requirements applicable to the project and this layer's scope. Page language or location alone does not establish a target. A whole-assembly requirement is not necessarily the R of one extra insulation layer; first establish the missing resistance and other components.
Should I round the result to a board thickness?
Check available variants and their declared R at that thickness, then verify the requirement with the appropriate method. This tool gives nominal thickness; it does not approve combining boards, installation, moisture performance or whole-assembly properties.
Why not enter R-13 directly as SI R?
US R-13 is about 2.289 m²·K/W, not 13 m²·K/W. Choose the R unit in the form. Reusing the same number would overstate nominal thickness by about 5.678 times.
Check thermal product data · Calculate an existing layer's R
NIST SP 1038, §5.3.5–5.3.6 — k and U unit conversion source on the IT Btu basis; examples are our own calculations.