From the label (bag, roll or bundle) for your R-value.
Extra for waste (%)
Enter measurements above 0 to see results.
Estimates only. Check measurements, product labels and local building codes before you buy or build.
How the Insulation Calculator works
Enter the area you are insulating and the coverage printed on the package, and the calculator tells you how many bags, rolls or bundles to buy. Use Attic / floor for a ceiling or floor (length × width), or Walls for a run of wall with the doors and windows taken out.
Area = length × width for an attic or floor, or total wall length × wall height minus doors and windows for walls. Add the waste allowance (10% by default), divide by the square feet one package covers, and round up to whole packages.
Example: a 30' × 24' attic is 720 sq ft. With 10% waste that is 720 × 1.10 = 792 sq ft. If the label says one bag covers 40 sq ft at your R-value, 792 ÷ 40 = 19.8, so buy 20 bags.
Coverage depends on the product and the R-value: a bag of blown-in insulation covers fewer square feet at a higher R-value because the layer is deeper, so read the chart for your target R-value, not the biggest number on the bag. Minimum R-values for new work are set by climate zone in the energy code (IRC section N1102, IECC section R402); local codes may differ, and the calculator does not check them.
Frequently asked questions
How many bags of blown-in insulation do I need?
Measure the attic floor (length × width), add about 10% and divide by the coverage per bag for the R-value you want, read from the chart on the bag. For a 30' × 24' attic and a bag that covers 40 sq ft, that is 792 ÷ 40 = 19.8, so 20 bags. A higher R-value means a deeper layer and fewer square feet per bag.
What R-value insulation do I need for my attic?
It depends on your climate zone. Minimum values for new construction are in the energy code (IRC section N1102, IECC section R402), and ENERGY STAR publishes recommended levels for existing homes; colder zones call for more. Your local code may differ, so check with your building department before you buy.
Do I subtract windows and doors when insulating walls?
Yes. In Walls mode, enter the number of doors and windows and their sizes (they start at 3' × 6' 8" and 3' × 4' and can be changed), and the calculator takes them out of the wall area. A 3' × 6' 8" door is 20 sq ft and a 3' × 4' window is 12 sq ft.
How much extra insulation should I buy for waste?
The calculator adds 10% by default to cover trimming batts around wiring, blocking and odd-sized stud bays. An open, rectangular attic may need less; a wall with many short or narrow bays may need more. Change the percentage to suit the job.