Insulation Calculator for Attics and Walls: Rolls, Batts, Bags and Cost

Estimate how much building insulation to buy for an attic, wall, ceiling or other rectangular surface. Enter the package coverage printed for the required R-value, or calculate full batts and boards from their dimensions.

Insulation Calculator

For loose-fill, use the bag coverage stated for your selected installed R-value.

Currency follows the price you enter.

Insulation estimate

Net surface
Area with extra
Full pieces needed
Whole packages
Purchased coverage
Coverage above adjusted area
Estimated material cost


How the insulation calculator works

First calculate the net surface area. For a rectangle, net area = length × width − openings. The calculator then multiplies net area by 1 + extra percentage. Whole packages are always rounded up.

For rolls, loose-fill bags or products with labeled coverage:

Packages = ceiling(area with extra ÷ coverage per package)

For batts or rigid boards:

Pieces = ceiling(area with extra ÷ piece area)
Packages = ceiling(pieces ÷ pieces per package)

Worked examples

  1. A 40 × 25 ft attic is 1,000 ft². With 10% extra and 40 ft² per roll, the result is 28 rolls, covering 1,120 ft².
  2. A 30 × 8 ft wall with 40 ft² of openings has 200 ft² net. With 10% extra, 8 × 1.333 ft batts and 10 batts per package, the result is 21 batts or 3 whole packages.
  3. A 12 × 8 m surface with 8 m² of openings has 88 m² net. With 5% extra, 1.2 × 0.6 m boards and 8 boards per package, the result is 129 boards or 17 packages.
  4. A known 1,500 ft² attic with 15% extra requires 21 loose-fill bags when the product label states 85 ft² per bag at the chosen R-value.
  5. A known 75 m² ceiling with 8% extra requires 7 rolls when each roll covers 12 m².
  6. A 200 ft² area with 10% extra requires 7 full 4 × 8 ft boards. If four boards come in a package, buy 2 packages.

What to enter for each insulation product

  • Batts and rigid boards: use the installed face dimensions and the number of pieces in each package.
  • Rolls: use the coverage printed on one roll.
  • Loose-fill or blown-in insulation: use the manufacturer’s coverage per bag for the selected installed thickness or R-value. Coverage changes with density and target R-value.

Limits and planning notes

This tool estimates quantity from coverage; it does not select an R-value, verify local energy-code compliance, calculate heat loss, predict energy savings or account for compression, framing, irregular cavities, inaccessible areas and installation technique. Confirm product compatibility, vapor-control requirements, fire clearances and package coverage with the manufacturer and local requirements. Measure different surfaces separately when their dimensions or products differ.

Related calculators

Use the Square Footage Calculator for multiple rooms or irregular totals. For framing quantities, use the Stud Calculator. For wall finish quantities, use the Drywall Calculator.