How Many Insulation Batts Do I Need?
Figuring out how many insulation batts you need is simple once you know your area and batt size. Enter the total square footage you want to insulate, pick a batt type (or set your own coverage per batt), and the calculator instantly tells you the exact number of batts required plus how many to buy with a waste allowance.
Enter your numbers
Picking a batt type fills this in; you can type your own value to override it.
Your results
| Item | Amount |
|---|---|
| Area to insulate | – |
| Batt type | – |
| Coverage per batt | – |
| Exact batts (area ÷ coverage, rounded up) | – |
| Extra batts for waste estimate | – |
| Batts to buy (incl. waste) | – |
| Total coverage of batts to buy | – |
Exact batts vs. batts to buy
Counts include a 10% waste allowance for cuts, offcuts, and damaged batts. This is a planning estimate — confirm coverage per batt on the manufacturer's label before ordering.
How to use this calculator
First measure your total area in square feet — for an attic floor multiply length by width; for walls multiply each wall's length by its height and add them together. Type that number into the "Area to insulate" field.
Next, pick your batt type. Each preset fills in the real coverage per batt: a 15″ × 93″ batt covers 9.69 sq ft, a 23″ × 93″ batt (for 24-inch stud spacing) covers 14.85 sq ft, and a 16″ × 48″ batt covers 5.33 sq ft. If your batts are a different size, choose "Custom" or just type your own coverage per batt — the rule is width times length in inches, divided by 144.
Set your waste allowance (10% is the standard default; use 15% for spaces full of outlets, pipes, and odd corners). Results update instantly as you change anything.
How it works
Exact batts = ceil(area ÷ coverage per batt). Division gives a fractional count, and the ceiling rounds up because you can only buy whole batts.
Batts to buy = ceil(exact batts × (1 + waste÷100)). The waste multiplier covers cuts around studs, outlets, and pipes plus the occasional damaged batt, and we round up again so you never come up one batt short.
Total coverage = batts to buy × coverage per batt — the square footage your purchase actually covers, which will always be a little more than your area because of rounding up.
These are planning estimates, not a professional materials takeoff: actual batt sizes vary slightly by brand, and unusual framing (double top plates, cathedral ceilings, kneewalls) changes real-world use.
Frequently asked questions
How do I measure the area I need to insulate?
Measure the length and width (or height) of each surface in feet, multiply them to get square feet, and add all the surfaces together. For walls, measure each wall's length times its height and add them up; for an attic floor, use length times width. Type the total into the calculator.
How much square footage does one insulation batt cover?
It depends on the batt size. A 15-inch by 93-inch batt covers about 9.7 square feet, a 23-inch by 93-inch batt covers about 14.9 square feet, and a 16-inch by 48-inch batt covers about 5.3 square feet. The rule is simple: multiply width times length in inches, then divide by 144.
Should I add extra batts for waste?
Yes. Cutting around studs, outlets, pipes, and odd corners creates offcuts, and a batt or two may arrive damaged. The calculator defaults to a 10% waste allowance, which is standard for batt insulation; raise it to 15% if your space has lots of obstructions.
What is the difference between faced and unfaced batts?
Faced batts have a paper or foil kraft facing that acts as a vapor retarder; they are typically installed with the facing toward the warm side of the wall (the interior in most climates). Unfaced batts have no vapor retarder and are common for attics or when a separate vapor barrier is used. The calculator works the same for both — only the coverage per batt changes.
Can I mix different batt types in one area?
It is best to keep one R-value and thickness per continuous area so the insulation fits snugly and performs as labeled. If you must mix types, measure and calculate each area separately, then add the results. Never compress a thick batt into a shallow cavity — compressed insulation loses R-value.