Insulation Calculator (R-Value, Thickness and Batts Needed)

Insulation is rated in R-value per inch, and the total R of a cavity is that rate times the thickness. This calculator gives the R-value of a thickness you have, or the thickness needed for the R-value you want, for batts, blown-in, rigid foam and spray foam using the U.S. Department of Energy figures. Then it counts the packages for your area from the coverage printed on the bag.

Insulation R-value calculator

How it works

R-value = R per inch × thickness; thickness = target R ÷ R per inch
R per inch (DOE): fiberglass batt 3.1, blown fiberglass 2.2 to 2.8, mineral wool 3.1 to 3.2, cellulose 3.7, EPS board 4.0, XPS 5.0, polyiso 6.0, open-cell spray foam 3.5, closed-cell 6.2
Packages = area × (1 + waste) ÷ coverage per package, rounded up
RSI (metric) = R × 0.176

Worked example

Standard 3-1/2 in fiberglass batts in 2×4 walls, 1,000 sq ft of wall, 40 sq ft per bag:

Insulation: Fiberglass batt (R-3.1 per in)
R-value per inch: 3.1
I know the: Thickness, and want the R-value
Thickness: 3.5 in
Area to insulate: 1000 sq ft
Coverage per package: 40 sq ft
Waste: 5%

The calculator gives:

R-value: R-10.9 (RSI 1.91) at 3.5 in
Packages: 27 at 40 sq ft each, with 5% waste
Area: 1,000 sq ft (92.9 m²)
Rate used: R-3.1 per inch

R-10.9 nominal (sold as R-11 or, in a denser batt, R-13) and 27 bags with 5% waste. To reach R-38 in an attic with blown cellulose at R-3.7 per inch you need about 10-1/4 in of depth; the bag lists how many square feet it covers at that depth.

Material and waste notes

DOE recommends roughly R-13 to R-21 for walls and R-30 to R-60 for attics depending on climate zone; the local energy code sets the minimum.
Compressing a batt into a shallower cavity lowers its R-value. Match the batt to the framing depth.
Framing takes about 15 to 25% of a wall’s area and insulates far worse than the cavity, so the whole-wall R is lower than the cavity R.
Blown products list coverage per bag at each depth; use that figure for the package coverage.
Vapour retarders, air sealing and ventilation matter as much as the R number; foam board and closed-cell foam also act as vapour retarders.

Frequently asked questions

What is the R-value of 3.5 inches of fiberglass?

About R-11 at 3.1 per inch; high-density 3-1/2 in batts are rated R-13 to R-15.

How many inches of blown insulation for R-38?

About 10 to 10-1/2 in of cellulose (R-3.7 per inch) or 13 to 14 in of blown fiberglass. Bags state the settled depth for R-38.

How many batts are in a bag?

Typically 8 to 16 batts, covering 40 to 90 sq ft depending on width, length and thickness. The coverage is on the label.

Which insulation has the highest R-value per inch?

Closed-cell spray foam and polyiso board, around R-6 per inch, followed by XPS at R-5. Fiberglass and mineral wool are about R-3.1 to 3.2.

What R-value should attic insulation be?

R-30 in the warmest zones up to R-49 or R-60 in cold climates. Add to existing insulation rather than replacing it.

Sources

Oncor Electric Delivery, "Insulation R-values per inch" (source: U.S. Department of Energy): fiberglass batt 3.1, mineral wool 3.1, cellulose 3.7, polystyrene board 4.0, polyurethane board 6.0, open-cell spray foam 3.5, closed-cell 6.2

Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.