Blown-In Insulation Bags Calculator
Enter your attic floor area and the coverage per bag from your product label at the R-value you want, and this tool rounds up to the number of bags to buy.
Calculator
Covering 1,000 sq ft at 42.7 sq ft per bag (from your bag label at the target R) takes 24 bags. Coverage per bag drops as target R rises — read your label.
Every bag of loose-fill insulation prints a coverage chart: how many square feet one bag covers at each target R-value, plus the minimum installed depth and the number of bags per 1,000 square feet. Coverage falls as target R rises, because a higher R means a thicker, denser layer. That is why this calculator asks you to read the coverage-per-bag at your target R off the label rather than guessing — the same product covers far more area at R-30 than at R-60.
Give it your attic floor area and that label figure and it divides, then rounds up to whole bags. Buying by the exact number keeps you from a second trip to the store mid-job — or a stack of returns.
Formula
bags = ⌈ area ÷ coverage-per-bag ⌉
The result is rounded up (the ⌈ ⌉ brackets) because you cannot buy a fraction of a bag. Coverage-per-bag is the value from your product's bag-count table at the R-value you are targeting — not a fixed constant, which is why the tool takes it as an input.
Worked example
For a 1,000 sq ft attic and a bag that covers 42.7 sq ft at R-38:
1,000 ÷ 42.7 = 23.4 → 24 bags
Target a higher R-49 with the same product and coverage-per-bag drops (say to about 33 sq ft), so the bag count climbs — read the exact figure off your label and re-run the numbers.
Settling and buying tips
Blow to the installed depth, not the poured height. Loose fill settles, so the coverage table assumes a settled thickness. Use the depth markers (thin rulers stapled to the trusses) that come in the machine kit and keep blowing until the whole attic reads the target depth.
Buy a bag or two extra for the corners and for topping off low spots — most stores refund unopened bags, and machine rental is often free with a minimum bag purchase. To convert a target R into the depth those bags should reach, use the insulation-depth calculator.
Frequently asked questions
How many bags of insulation do I need for 1,000 square feet?
Divide 1,000 by the coverage-per-bag on your product label at your target R, then round up. At 42.7 sq ft per bag (a typical R-38 figure) that is 24 bags; at a higher R the coverage per bag drops and you need more.
Why does coverage per bag change with R-value?
A higher R-value means a thicker, denser layer, so one bag spreads over fewer square feet. Bag labels list coverage separately for each target R for exactly this reason — always read the row that matches the R you want.
Do I need to account for settling?
The bag's coverage chart already assumes the settled thickness, so if you install to the labeled installed depth you are covered. Blow slightly high only if the instructions give a separate initial-installed height above the settled depth.
Can I rent the blower?
Most home centers lend an insulation blower free or cheaply when you buy a minimum number of bags. Have a helper feed bags while you run the hose, and keep depth rulers visible so you hit the target loft evenly.