How to use
- Measure the net area, after you subtract openings you will not fill.
- Read the bag for the coverage at the thickness you are installing, and type that square footage.
- Set waste for cuts around framing. Ten percent is only a starting value.
- Press Calculate and buy whole bags.
How it's calculated
Area to buy = net area × (1 + waste ÷ 100).
Bags = that area ÷ coverage per bag, rounded up. R-value is a label on the product. It is not an input, because this page is a count, not a heat-loss model.
Worked example
1,000 square feet net, a bag that covers 40 square feet, and 10 percent waste: the area to buy is 1,100. Bags = 1,100 ÷ 40 = 27.5, which rounds up to 28 bags. A bag rated for 80 square feet at a thinner install would cut that count about in half.
Assumptions
One coverage number for the whole area. Mixing batts and blown fill means two runs. The page does not know stud spacing.
FAQ
Why can't I enter R-30 and get a count?
R-30 on two products can mean two different covered areas per bag. The bag's coverage table is the number that orders correctly.
Do I subtract windows?
Yes, if you will not insulate that opening. Enter the area you will actually fill.
What about compression?
If you compress a batt, the coverage and the labeled R-value both change. Use the coverage that matches how you will install it.
Is waste always 10 percent?
No. A simple attic bay may need less. Lots of blocking may need more.