Insulation Calculator
Bag counts for fibreglass batts. Enter the area — wall length × height for walls (openings not subtracted), or length × width for an attic floor. The ticket lists bags for each common R-value because coverage per bag drops as the batts get thicker. Blown-in is figured differently; see the FAQs.
Order 4 R-13 bags — bag coverage is printed on the bag — divide the square feet to cover by it for any product.
- Area
- 320 ft²
- Waste / cuts 5%
- + 16 ft²
- Coverage to buy
- 336 ft²
- Metric
- 29.7 m²
- R-13 kraft batts, 15″ × 93″ (≈ 97 ft²/bag)
- 4
- R-15 batts, 15″ × 93″ (≈ 68 ft²/bag)
- 5
- R-19 batts, 15″ × 93″ (≈ 49 ft²/bag)
- 7
- R-30 batts, 15″ × 48″ (≈ 58 ft²/bag)
- 6
- R-38 batts, 15″ × 48″ (≈ 43 ft²/bag)
- 8
- Materials + delivery$45–$85 / bag · Sep 2026
- $180 – $340
- Installed (with labor)typ. $560
- $340 – $900
Quantities follow the ticket. Links may earn us a commission at no cost to you; they never change a number.
What will this insulation job cost?
See the installed price range for these quantities and get up to 3 quotes from local contractors. Free, no obligation.
How to calculate insulation by hand
Bags = Area (ft²) × (1 + waste) ÷ coverage per bag (from the bag label)
Worked example
- Wall area: 40 ft × 8 ft = 320 ft²; with 5% waste, 336 ft².
- R-13 at 96.9 ft² per bag: 336 ÷ 96.9 = 3.5 → 4 bags.
- R-19 (2×6 walls) at 48.9 ft² per bag: 7 bags.
- Attic at R-38 (42.7 ft² per bag): 8 bags for the same 320 ft².
- 2×4 wall
- R-13 or R-15 (3½″)
- 2×6 wall
- R-19 or R-21 (5½″)
- Attic, most of the US
- R-38 to R-49
- Batt width
- 15″ for 16″ o.c. · 23″ for 24″ o.c.
Questions crews ask
How many bags of insulation do I need?
Divide the area by the coverage printed on the bag and round up: a 40 × 8 ft wall (320 ft²) takes 4 bags of R-13 (97 ft² each) or 7 bags of R-19 (49 ft²). The calculator lists the common R-values so you can pick the one for your framing.
What R-value do I need?
Walls: R-13 to R-15 in 2×4 framing, R-19 to R-21 in 2×6. Attics: R-38 in the South, R-49 to R-60 in the North (DOE zones 5–8). Floors over unheated space: R-25 to R-30. Local code sets the minimum.
How much does a bag of insulation cover?
It depends on thickness: R-13 about 97 ft², R-15 about 68, R-19 about 49, R-30 about 58, R-38 about 43 — for 15″ batts. 23″ batts for 24″ framing cover more per bag. Read the label; it is printed on every bag.
Should I subtract windows and doors?
For walls, subtract large openings — a 2-car garage door or a wall of glass — but leave normal windows in; the offcuts around them are the waste. For attics, subtract nothing.
Batts or blown-in for an attic?
Blown-in cellulose or fibreglass fills around wiring and odd joist bays better and is cheaper per R-value in an attic; batts are easier for a DIY wall job. Blown-in is sold by the bag with a depth chart on the label — R-38 takes roughly one bag per 40 ft² of cellulose.