How it works
Wall size, stud spacing, and the coverage printed on the bag.
Measure the wall
Length and height in feet. Add up every wall in the room and enter the total length.
Set the stud spacing
16 inch centres is standard framing; 24 inch shows up in newer builds and garages.
Get bags and cavities
How many bags to buy, and how many cavities you are filling.
Buy for the coverage, not the square footage
Insulation is one of the few materials where the label does the work for you. Every bag prints its coverage in square feet, and that figure already accounts for the batt width and length inside. So the calculation is simply your wall area divided by that number — which is why this calculator asks you to enter it rather than guessing.
Where people go wrong is assuming the coverage is fixed. It is not. A bag of R-13 for a 2x4 wall covers far more square footage than a bag of R-21 for a 2x6, because the thicker batt takes more material per square foot. Two bags of the same brand can differ by a factor of two. Read the bag.
Batt width matters too, and it is not the same as your stud spacing. Batts for 16 inch centres are 15 inches wide, and batts for 24 inch centres are 23 inches. The extra inch is deliberate — it lets the batt friction-fit against the studs and stay put without stapling. Buying 23 inch batts for 16 inch framing means cutting every single one lengthwise.
The cavity count is there as a sanity check. If the number of cavities looks wrong for the wall you are standing in front of, your length or spacing is off, and the bag count will be wrong too. And do not compress insulation to make it fit: a batt squashed into a narrow bay loses much of the R-value you paid for.
Frequently asked questions
How many square feet does a bag of insulation cover?
It depends entirely on the R-value and thickness — anywhere from about 40 to over 100 ft² per bag. The figure is printed on the bag, so use that number rather than an average.
What width batt do I need?
15 inches for framing on 16 inch centres, and 23 inches for 24 inch centres. The batt is deliberately an inch wider than the gap so it friction-fits.
Does this include the attic?
Enter the attic floor as length by width and it works the same way, but attics are usually done with blown-in insulation sold by depth rather than batts.
Can I compress insulation to fit?
No. Compressing a batt into a smaller cavity reduces its R-value. Cut it to fit instead, or buy the correct width.
More material calculators
Estimates are for planning only. Confirm quantities with your supplier before ordering.