Estimate insulation coverage — square footage and number of bags or batt packs — from the area you need to cover.
buy = area × (1 + waste); bags = buy ÷ bag_coverage
Add waste to the area, then divide by the coverage stated on the insulation bag or batt pack.
Coverage varies by R-value and material — always confirm the square footage per bag on the product.
For 500 square feet of attic at the coverage printed on the bag — say 40 square feet per bag for a given R-value of blown-in insulation — you'd need 500 ÷ 40 = 12.5 bags, and with 10% waste about 14 bags.
Coverage is everything here and it changes with R-value: the same bag covers far less area at R-49 than at R-30 because you're applying it deeper. Always read the coverage chart on the specific product for your target R-value rather than assuming.
Coverage depends on the specific product and R-value — use the figure on the package for an accurate count.
Before any quantity, fix the target R-value — the thermal resistance your assembly needs. It is set by climate zone and by what you are insulating: attics need the most (often R-38 to R-60), 2×4 walls around R-13 to R-15, 2×6 walls R-19 to R-21. A higher R-value means thicker batts or deeper blown-in, which directly changes how much you buy. Confirm the requirement against current local code.
Each type is quantified differently. Batts are sized to 16-in or 24-in stud spacing and bought by the package, each stating its coverage at a given R-value. Blown-in is sold by the bag, and its coverage falls as the target R-value rises — read the bag's coverage chart at your actual R-value. Spray foam is measured in board-feet (one square foot one inch thick), with closed-cell about R-6.5 per inch and open-cell about R-3.7.
By square foot of coverage per bag or batt pack, which varies by R-value and brand.
Divide the attic area by the square feet a bag covers at your target R-value, then round up. Coverage drops as R-value rises, so always read the chart at your actual target, not the maximum coverage on the front of the bag.
Attics commonly target R-38 to R-60 depending on climate zone. Check your local code, since minimums vary by region and are periodically updated.
For tight estimates you can subtract the framing fraction, since studs and joists occupy roughly 15% of a wall's face, but most estimators leave it in as part of the buffer.