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.
By square foot of coverage per bag or batt pack, which varies by R-value and brand.