About this tool
The Insulation Batt & Roll Calculator estimates how many batts, rolls or bags of loose-fill insulation you need to cover a wall or attic to a target R-value, and the nominal thickness that R-value requires. Enter the area as a length and width in feet, subtract any window or door openings, choose the target R-value, the stud or joist spacing, and the product type. Coverage per bag defaults from manufacturer tables but you can override it. All math runs locally in your browser.
Net area is the gross area minus openings. For framed cavities the tool applies a frame factor — studs at 16" on-centre occupy about 10% of the wall (usable ≈ 0.90) and 24" o.c. about 15% (usable ≈ 0.85) — so batts are only bought for the cavity space they actually fill. Bags or rolls equal the usable area divided by coverage per bag, multiplied by the waste factor and rounded up. Loose-fill for an attic covers the whole net area (no frame factor) and its required depth is the target R-value divided by R-per-inch (fiberglass ≈ 2.5, cellulose ≈ 3.5).
The recommended thickness follows standard product lines: R-13 and R-15 fit a 3.5" 2×4 wall, R-19 needs ~6.25", R-21 ~5.5", R-30 ~8.25", R-38 ~12" and R-49 ~15.5". Use those figures to confirm your cavity or attic depth can actually hold the R-value you are targeting before you buy.