The Formula Behind This Calculator
Area = length x width (sq ft)
Batts and rolls: units = round up (area / coverage per package or roll from the label)
Blown: units = round up (area / coverage per bag at the target R value)
Depth guide: fiberglass about R-3.2 per inch blown, cellulose about R-3.7 per inch
Insulation is sold by coverage, and coverage is honest only at a stated depth. A bag of blown fiberglass that covers 85 square feet at R-19 covers only about 54 at R-30 and 32 at R-49, because the same bag is being spread thinner or thicker to hit the target. That is why this calculator refuses to estimate blown insulation without an R value: an area alone is only half the question. Batts and rolls avoid the issue by being manufactured to a fixed thickness and R value, so their label coverage can be used directly. The depth line in the results is a field check: after blowing, a ruler pushed to the deck should read at least the depth shown, everywhere, not just near the hatch.
Worked Example: A 30 by 40 Attic to R-30 in Blown Fiberglass
An attic over a 30 by 40 foot house, 1,200 square feet, is being topped up to R-30 with blown fiberglass, the most common attic upgrade in the country.
- Area: 30 x 40 = 1,200 square feet.
- Coverage per bag at R-30: about 54 square feet (roughly 9.5 inches of settled depth).
- Bags: 1,200 / 54 = 22.2, round up to 23 bags.
- Depth check: R-30 blown fiberglass settles near 9.5 inches, so mark a ruler or attic ruler cards at 10 inches before you start and blow to the marks.
- Budget: 23 bags at $55 to $75 is about $1,265 to $1,725 in product, and most home centers waive the blower rental at this quantity, October 2026 national pricing.
Run the same attic as blown cellulose and the count jumps to the low fifties of bags, because cellulose is denser and each bag carries less coverage. The bags are far cheaper each, so the totals end up closer than the counts suggest. That trade, many light bags against fewer heavy ones, is a real decision when every bag goes up a hatch on your shoulder.
A Complete Guide to Estimating Insulation Quantities
Insulation estimates go wrong in a predictable way: someone buys for the area and forgets that R value is a thickness, not a label. The result is an attic blown to R-19 with bags bought for R-30 money, or the reverse. This guide covers how coverage, depth and product choice connect, and the prep work that decides whether the insulation performs at its rating.
Coverage Charts and Why They Shrink With R Value
Every blown insulation bag carries a coverage chart: rows of R values against the square feet one bag covers and the minimum depth to maintain. The physics is simple. A bag holds a fixed weight of fiber. Spread it at 6 inches and it covers a wide area at about R-19. Spread the same bag at 10 inches and the area shrinks while the R value grows to about R-30. The calculator uses mid chart values from the major manufacturers for both fiberglass and cellulose, but your bag's own chart governs on the day: if it differs, the honest fix is to divide your area by the chart coverage for your R value, which is all this tool is doing. Note the settled depth on the chart too. Blown products settle a little in the first year, and the chart depth already accounts for it.
Measuring Attics and Walls Honestly
For an attic, measure the ceiling footprint from inside if you can do it safely from the hatch with a laser measure, or use the outside wall dimensions minus a foot for wall thickness. Sloped ceilings and knee wall attics need each surface measured separately. Skip nothing for the rooms under vaulted sections, because those ceilings are exactly where heat escapes. For walls, measure length times ceiling height per wall and subtract doors and windows at roughly 21 and 15 square feet, the same deductions used across this site. Then match the product to the framing: batt width follows stud spacing, batt thickness follows stud depth, and no estimate can rescue a batt forced into the wrong cavity.
What Insulation Costs in 2026
As of October 2026, based on national home center pricing, a package of R-13 batts covering about 106 square feet runs $55 to $95 depending on facing and width, and a roll of R-19 runs $22 to $38 for about 49 square feet. Blown fiberglass bags run $55 to $75 each with the blower rental commonly free at 10 or more bags, and blown cellulose runs $11 to $17 per bag. Professional installation from published surveys runs about $1.50 to $2.50 per square foot for an attic blow in, materials included, so the 1,200 square foot example attic quotes around $1,800 to $3,000 installed against roughly $1,300 to $1,700 in DIY product. Air sealing supplies for the same attic, foam, caulk and baffles, usually add $75 to $200 and are the best money in the project.
Prep and Safety That Change the Result
- Air seal first: foam around pipes, wires and top plates, weatherstrip the hatch, and cover non IC rated recessed lights with rated covers or keep insulation clear per the fixture label.
- Install soffit baffles at every vented bay so the new depth does not choke attic ventilation at the eaves.
- Set depth rulers or marked cards every few feet and blow to the marks, starting farthest from the hatch and working back.
- Wear a proper respirator, eye protection and gloves, and lay planks across joists to move safely. Never step on the ceiling drywall between joists.
- Photograph the finished depth at several rulers before you close the hatch. If a contractor did the work, those photos are your verification.