How insulation batt calculator works
Batt insulation is sold by width and R-value, not by square footage alone. This calculator starts from your area — wall bays between studs, or an attic floor — then matches the two specs that decide what you buy: nominal batt width, 16 or 24 inches to match standard stud and joist spacing on center, and the R-value you're targeting, which sets an approximate thickness from R-11 (3½ in) up to R-49 (15 in).
Coverage math is simple: a 16 in × 8 ft batt covers (16 ÷ 12) × 8 = 10.67 sq ft, and a 24 in wide batt covers 16 sq ft. The tool divides your area plus a 10% overage by that figure and rounds up to whole batts. Attic jobs usually run from rolls instead — a 16 in × 40 ft roll covers about 53 sq ft — so the roll mode divides by roll coverage the same way.
The result includes nominal thickness so you can check depth fit before buying: a 2×4 wall cavity is 3½ in deep (R-13 or R-15 max), a 2×6 wall takes R-19 or R-21, and attic R-38 to R-49 needs 12–15 inches of clearance. Nominal widths ignore the slight friction-fit compression inside real bays, so treat counts as estimates — the overage absorbs off-cuts around electrical boxes, wire runs and attic hatch framing.