Count the coverage in the bag you will buy
R-value describes thermal resistance, not package coverage. Two bags labelled R-13 can contain different batt widths, lengths or quantities. Match the selected pack to its label before using the result.
Why this pack covers 106.56 square feet
The selected example pack contains eleven batts, each 15 × 93 inches. One batt covers (15 ÷ 12) × (93 ÷ 12) = 9.6875 square feet. Eleven cover 106.5625 square feet. A thirteen-piece pack of the same size would cover 125.9375 square feet and requires a different bag count.
A 12-by-12 room
Four 8-foot-high walls total 384 square feet. Deducting the tool's assumed door and window areas of 35 square feet leaves 349. With 10% extra for this worked example, the purchase area is 383.9 square feet: 383.9 ÷ 106.5625 = approximately 3.60 bags, rounded up to four.
Four bags provide 44 batts. Before buying, compare the number and height of actual stud cavities with those batt dimensions. Openings, short cavities and offcut reuse can make a simple area estimate differ from the final cutting list.
Make three checks before ordering
- Match batt width to the cavity and framing spacing.
- Confirm the required R-value, thickness and facing for your assembly and location.
- Check the piece count and coverage printed on the actual package.
This tool estimates a selected product quantity; it does not determine your building-code requirement or vapor-control design. Price the matching pack locally and add any other required installation materials separately. For loose-fill attic insulation, use the blown-in calculator.