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Insulation Calculator

Work out how thick your insulation needs to be to reach a target R-value, and how much to buy for the area. Pick fiberglass, cellulose, spray foam or foam board and get the thickness, volume and package count.

Quick answer: An attic of 1,000 sq ft needs fiberglass batts about 11.88 inches thick to reach R-38, which is 28 packages of 40 sq ft with 10% extra.

Your measurements

ft
ft
sq ft
R
%
sq ft

Thickness needed

11.88inches

Area to insulate
1,000 sq ft
Thickness for R-38
11.88 inches
Volume with 10% extra
1,088.5 cubic feet
Area to buy with extra
1,100 sq ft
Packages (40 sq ft each)
28 packages

R-value per inch is typical and varies by brand and density. Check the label for the real figure, and do not compress batts, because that lowers their R-value.

How to use this calculator

  1. Enter the length and width of the area, or the total area if you know it.
  2. Subtract the area of any windows, hatches or other gaps if they are included.
  3. Choose the type of insulation you plan to use.
  4. Enter the R-value you are aiming for. Your local energy office or building code lists the recommended value for your climate.
  5. Enter the coverage printed on one package to see how many packages to buy.

How the numbers are calculated

R-value measures resistance to heat flow, and each material has a typical R-value per inch. The thickness you need is the target R-value divided by the R-value per inch of the material you picked.

The calculator multiplies the net area by that thickness in feet to get the volume, then adds your waste allowance. It divides the area, with waste, by the coverage of one package and rounds up to give a package count.

Worked example

An attic of 40 ft by 25 ft has 1,000 sq ft. For R-38 in fiberglass batts at about R-3.2 per inch, you need 11.88 inches. With 10% extra the volume is about 1,088.5 cubic feet. If a package covers 40 sq ft, you need 28 packages.

Tips before you buy

  • Recommended R-values depend on the climate zone and the part of the house, so check your local guidance before you buy.
  • Do not squash batts into a smaller space. Compressed insulation loses R-value.
  • Seal air leaks around wires, pipes and the attic hatch before you add insulation. Air movement can defeat the insulation.
  • Keep insulation clear of recessed lights and chimney flues unless the products are rated for contact.
  • Wear gloves, a mask and eye protection when handling fiberglass or blown insulation.

Frequently asked questions

How thick is R-30 insulation?

Fiberglass batts at about R-3.2 per inch need about 9.4 inches to reach R-30. Closed-cell spray foam needs only about 4.6 inches.

What R-value do I need in my attic?

It depends on your climate. Colder areas need a higher value. Check the recommendation from your local energy office or building code.

Does the calculator include framing?

No. Studs and joists take up part of the area, so subtract that space yourself if you are only filling the gaps between them.

Why do packages list different coverage?

Coverage depends on the thickness of the product. Use the figure printed for the R-value you are buying.

Quick answers

Related calculators

These results are estimates for planning purposes. Material sizes, densities and local requirements vary, so confirm quantities with your supplier or a qualified contractor before ordering.

Last updated October 11, 2026.