Insulation Calculator
Choose batts or blown-in, enter the area, and get the number of bags to buy. Batt counts use the coverage printed on the bag; blown-in counts use the manufacturer's coverage chart for your R-value.
- Manufacturer coverage charts
- Walls, floors and attics
- Feet or metric
2 What to buy
Updates as you typeThis uses Owens Corning's kraft-faced bag. Unfaced bags and other brands often hold fewer batts (R-13 15" × 93": 13 kraft-faced vs 11 unfaced), so enter the "sq ft per bag" from your label above.
| Product | R-13 · 15" × 93" kraft (studs 16" o.c.) |
|---|---|
| Framed area | 320 sq ft |
| Cavity area (batts fill the bays, not the studs) | 300 sq ft |
| With waste | 315 sq ft |
| Coverage of the bags you buy | 378 sq ft |
Shopping list
Quick answer
A 40 ft run of 8 ft tall 2×4 walls is 320 sq ft. A 15-inch batt fills 15/16 of each 16-inch stud bay, so we count 94% of that, so you need 315 sq ft of R-13 with 5% for cuts: 3 bags at 125.94 sq ft per bag. For a 1,200 sq ft attic at R-49 you'd need 30 bags of AttiCat blown fiberglass (17 inches deep) or 85 bags of cellulose.
Blown-in bags for common attic sizes
| Attic floor | R-30 | R-38 | R-49 | R-60 |
|---|---|---|---|---|
| 500 sq ft | 8 / 20 | 10 / 27 | 13 / 36 | 16 / 43 |
| 800 sq ft | 12 / 31 | 16 / 43 | 20 / 57 | 26 / 69 |
| 1,000 sq ft | 15 / 39 | 19 / 53 | 25 / 71 | 32 / 86 |
| 1,200 sq ft | 18 / 47 | 23 / 64 | 30 / 85 | 38 / 104 |
| 1,500 sq ft | 22 / 58 | 29 / 79 | 38 / 107 | 48 / 129 |
| 2,000 sq ft | 30 / 78 | 38 / 106 | 50 / 142 | 63 / 172 |
Bare attic. AttiCat fiberglass / Greenfiber cellulose bags, from each maker's coverage chart. Use the calculator above for your exact size.
How to work out batts
Measure the surface you're insulating: wall length × height, or floor or attic length × width. Batts go between the studs or joists, not over the wood. A 15-inch batt is 15/16 of a bay framed 16 inches on center, so we count 94% of the wall area; a 23-inch batt in 24-inch framing is 23/24, about 96%. Add a little for cuts, then divide by the square feet printed on the bag. Bags differ by brand and facing, so the label figure is the one to use.
Worked example: 40 ft of 2×4 wall, 8 ft tall
- Wall area: 40 × 8 = 320 sq ft
- Cavity at 16" on center: 320 × 15/16 = 300 sq ft
- Add 5% for cuts: 315 sq ft of R-13
- At 125.94 sq ft per bag: 3 bags
Subtract large openings such as patio doors. Small windows can stay in; the offcuts fill short bays above and below them.
Which batt fits
| Framing | Batts |
|---|---|
| 2×4 walls (3½" deep) | R-11, R-13 or R-15 |
| 2×6 walls (5½" deep) | R-19 (slightly compressed) or R-21 |
| Floors over crawl spaces | R-19 to R-30, depending on joist depth |
| Attics | R-30, R-38 or R-49 attic batts, often in two layers |
R-19 is made 6¼ to 6½ inches thick. Squeezed into a 5½-inch 2×6 wall it performs at about R-17 to R-18, according to Johns Manville. R-21 high-density batts are made 5½ inches thick for 2×6 walls.
Blown-in attic insulation
Loose fill is sold by the bag, and every product prints its own chart: bags per 1,000 sq ft and the minimum thickness for each R-value. The calculator uses two common charts:
| R-value | AttiCat fiberglass (27.5 lb) | Greenfiber cellulose (25 lb) |
|---|---|---|
| R-19 | 9 bags / 1,000 sq ft, 7" deep | 22.6 bags / 1,000 sq ft, 5.7" installed (5.3" settled) |
| R-30 | 14.6 bags / 1,000 sq ft, 10.75" deep | 38.6 bags / 1,000 sq ft, 8.9" installed (8.3" settled) |
| R-38 | 19 bags / 1,000 sq ft, 13.5" deep | 52.6 bags / 1,000 sq ft, 11.2" installed (10.4" settled) |
| R-49 | 25 bags / 1,000 sq ft, 17" deep | 70.8 bags / 1,000 sq ft, 14.3" installed (13.3" settled) |
| R-60 | 31.5 bags / 1,000 sq ft, 20.5" deep | 86 bags / 1,000 sq ft, 17.3" installed (16.1" settled) |
Cellulose needs two and a half to three times as many bags as AttiCat for the same R-value, and it ends up thinner, because cellulose is denser. Cellulose settles a little, so blow it to the installed depth, not the settled one; AttiCat's maker says its settling is negligible. Staple depth rulers to the joists, marked at the installed depth.
Before you add insulation
- Air-seal first. Foam the gaps around plumbing stacks, wiring holes and the top plates of walls. Insulation slows heat, not air.
- Keep soffit vents open. Fit a baffle in each rafter bay above the soffit before blowing.
- Check recessed lights. Only IC-rated fixtures can be covered with insulation.
- Our climate zone lookup gives the code minimum for your county, and the R-value chart has every zone.
Frequently asked questions
How many bags of insulation do I need for 1,000 sq ft?
It depends on the product and R-value. For blown-in at R-49: about 25 bags of Owens Corning AttiCat fiberglass or 70.8 bags of Greenfiber cellulose. For R-13 wall batts: about 8 bags of Owens Corning 15" × 93" batts (125.94 sq ft each) for 1,000 sq ft of 2×4 wall.
How many square feet does a bag of R-13 insulation cover?
Owens Corning PINK Next Gen R-13 kraft-faced 15" × 93" batts cover 125.94 sq ft per bag, and the 23" × 93" size covers 148.54 sq ft. Unfaced R-13 15" × 93" bags hold fewer batts and cover 106.56 sq ft, the same as Johns Manville's. Always check the label.
Should I use batts or blown-in insulation in the attic?
Blown-in is faster on an open attic floor and fills around wires and odd framing without gaps. Batts are easier where you can't use a blowing machine or need to add a layer over joists in small sections. Blown-in only reaches its R-value with both the chart's bag count and its depth; batts reach theirs when they fill the cavity without being squashed.
Can I put new insulation on top of old insulation?
Yes, as long as the old insulation is dry and not moldy. Blown-in can go straight on top. Add unfaced batts (no paper facing) over existing insulation, laid across the joists, so you don't trap moisture between layers.
Last reviewed 2026-10-21. See how we calculate for our sources and assumptions.