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R Value Calculator

Calculate the R-value you get from a given thickness and the material's R per inch, including layers.

Measurement units

In inches.

From the product label; it varies by material.

Enter 0 if there is none.

Free, runs entirely in your browser, and your numbers never leave your device. Results are estimates for planning only.

What this calculator does

R-value is simply thickness times the material's R-per-inch, and layers add together. This tool turns a thickness and an R-per-inch into an R-value, and lets you add any existing insulation to find the total R-value of the assembly.

How to use it

  1. Enter the insulation thickness in inches.
  2. Enter the product's R-value per inch from its label.
  3. Add the existing R-value of the assembly (use 0 if none), then read the total R-value.

The formula

Added R = thickness (inches) × R-value per inch. Total R = existing R + added R.

Example calculation

6 inches of a product rated R-3.5 per inch, added to an existing R-0 surface:

  • Added R: 6 × 3.5 = 21
  • Existing R: 0
  • Total R: 0 + 21 = R-21

Result: R-21 from a 6-inch layer at R-3.5 per inch.

Buying and planning tips

  • R-values are additive, so layering products is a valid way to reach a target.
  • Use the R-per-inch on the label; it differs between fiberglass, mineral wool, and foam.
  • Remember that air films and other materials add small amounts of R as well.

Common mistakes to avoid

  • Mixing up R-per-inch with the product's total R-value.
  • Adding R-values of materials in parallel (like studs and cavity) as if they were in series.
  • Assuming the rated R-value holds even when the insulation is compressed.

Assumptions and limits

  • Added R = thickness × R-per-inch, with values you provide.
  • Layers are assumed to be in series (stacked), so their R-values add.
  • This is a planning estimate only, not engineering or building-code advice; verify product ratings on the label.

Frequently asked questions

How do I calculate R-value from thickness?

Multiply the thickness in inches by the material's R-value per inch. Six inches of an R-3.5 per inch product gives R-21.

How do I work out attic insulation R-value?

Enter the depth of insulation across your attic floor and its R-per-inch, then add any existing R-value. As a guide, many attics target around R-49 to R-60 depending on climate — check your local recommendation.

Do R-values add together?

Yes, when layers are stacked (in series). Add the existing assembly's R-value to the new layer's R-value for the total.

What is R-per-inch?

It is how much R-value one inch of a material provides. It varies by product — fiberglass, mineral wool, and rigid foam all differ, so use the label figure.

Does compression change the R-value?

Yes. Squashing insulation below its rated thickness lowers its R-value, so the calculated figure assumes it is installed at full loft.

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