Insulation Batts Count Calculator
Count how many insulation batts a wall needs from its length and height, stud spacing and batt length.
How it works
cavities = ceil(length×12 ÷ stud spacing) · batts = cavities × ceil(height ÷ batt length)
Batts fill the bays between studs, so the count starts with the number of cavities: the wall length in inches divided by the on-center spacing, rounded up. Each cavity needs enough batts to reach the wall height — one 8 ft batt per standard 8 ft wall, or more where walls are taller. Batts are also sold by coverage per bag, so cross-check the bag’s stated square footage against the wall area.
Worked example
A 40 ft × 8 ft wall framed 16 in on center: 40 × 12 = 480 in ÷ 16 = 30 cavities. An 8 ft wall takes one 8 ft batt per cavity, so 30 batts, covering the wall’s 320 square feet.
Frequently asked questions
How many batts fit in a bag?
It varies by R-value and width, so a bag lists its coverage in square feet rather than a fixed count. Use this batt count to know how many pieces you need, then divide the wall area by the bag’s coverage to know how many bags to buy.
What width batt for 16 vs 24 inch studs?
15-inch batts friction-fit 16-inch on-center framing and 23-inch batts fit 24-inch framing, because the stud spacing is measured center to center while the clear cavity is a bit narrower. Match the batt width to your framing so they seat without gaps or compression.
Do I cut batts around outlets and blocking?
Yes — batts should be split around wiring and notched around boxes so they fill the cavity without compressing, since compressed insulation loses R-value. Add a little extra for those cuts and for waste.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.