Insulation Calculator — how many bags or rolls

How it works

Insulation is sold by the bag (blown-in) or roll/batt (rolled), each labelled with the area it covers at a given thickness or R-value. Estimating comes down to area ÷ coverage, per Wolfram MathWorld's A = length × width:

1. Area — the square footage of the wall, floor or attic you are insulating.
2. Coverage per package — read it off the product; a bag of blown-in insulation covers a smaller area at a higher R-value, and batts cover the area of the wall cavities they fill.
3. Packages = area ÷ coverage, times (1 + waste %), rounded up to whole packages.

Because coverage changes with the R-value and product, the calculator takes it as an input rather than assuming a value. A waste allowance of about 10% covers trimming batts to fit and the extra depth blown-in insulation settles to; use less for a simple open attic, more for a wall full of studs and obstructions.

Worked example

For 500 ft² at 40 ft² per package, before waste you need 500 ÷ 40 = 12.5 packages. Adding 10% waste gives 12.5 × 1.1 = 13.75, rounded up to 14 packages — which cover 560 ft².

Frequently asked questions

How many bags of insulation do I need?

Divide the area by the coverage printed on the bag, add a waste allowance, and round up. For 500 sq ft at 40 sq ft per bag with 10% waste, that is 500 ÷ 40 × 1.1 = 14 bags.

Why isn't the coverage per package fixed?

Because it depends on the product and the R-value. A bag of blown-in insulation covers less area at a higher R-value, and batts are sized to specific cavity depths. The coverage is always printed on the packaging, so the calculator takes it as an input.

What is R-value?

R-value measures resistance to heat flow — higher means better insulation. Reaching a higher R-value with blown-in insulation means a thicker layer, so each bag covers a smaller area, which you enter as the coverage.

How much waste should I add?

About 10% is typical, to cover trimming batts around studs and the settling of blown-in insulation. A clear, open attic can use less; a wall full of framing and wiring may need more.

How do I measure the area for walls?

Multiply wall length by height for each wall and add them, then subtract large openings such as windows and doors. For an attic, use the floor area of the space below.

Bags or rolls — does the math change?

No. Whether the package is a bag of loose fill or a roll of batts, you divide the area by the coverage that package provides. Just enter the coverage figure for the product you are buying.