Attic Insulation Calculator
Enter your measurements
Results
- Bags needed49 bags
- Attic area1,200 sq ft
- Coverage per bag24.49 sq ft at R-49
- Fill depth18.8 in
Estimated cost
per sq ft of attic at R-49 (material)
Material cost is per sq ft of attic floor at R-49 for blown-in fiberglass; cellulose typically runs slightly lower per sq ft. Installed price includes professional labor with a blower setup and assumes a standard accessible attic — costs rise for tight access hatches, HVAC equipment in the way, or very low roof pitches.
Estimate only — prices vary by region, supplier, and season. Get a local quote before buying.
This attic insulation calculator tells you how many bags of blown-in insulation you need to hit a target R-value in your attic. Enter your attic's length and width, choose your R-value goal, and it returns total bag count, coverage per bag, and the required fill depth — the three numbers you actually need before loading up at the hardware store. It's built around the way blown-in fiberglass is really sold: each bag is rated for a specific coverage area at a specific settled depth, printed right on the label. The catch with blown-in fiberglass and cellulose is that coverage per bag drops as you add depth. A bag that covers about 40 sq ft at R-30 might only cover 24 sq ft at R-49, because reaching the higher R-value means piling the fill deeper. That shrinking-coverage relationship is what trips up manual estimates and back-of-envelope math. Standard attic bags run roughly 19 to 35 lbs each depending on product and target. Many big-box stores also lend a blower machine free once you buy a minimum number of bags, so nailing your bag count up front tells you whether you qualify and saves a second trip.
How it’s calculated
Bags = Attic area ÷ coverage per bag. Coverage falls as target R-value rises (more depth = less area per bag).
Worked example
For a 40 ft x 30 ft attic — 1,200 sq ft — targeting R-49, the calculator returns 49 bags at roughly 24.49 sq ft of coverage per bag, with a required settled fill depth of about 18.8 inches. That depth figure matters before you start: if your attic has soffit baffles or low-slope roof decking near the eaves, confirm you have clearance for nearly 19 inches of loose fill before buying, since the perimeter is usually the tightest spot.
Inputs
- Attic length
- 40 ft
- Attic width
- 30 ft
- Target R-value
- R-49
Result
- Bags needed
- 49 bags
- Attic area
- 1,200 sq ft
- Coverage per bag
- 24.49 sq ft at R-49
- Fill depth
- 18.8 in
- Estimated material cost
- $1,080 – $2,040
Materials & pricing near you
Blown-in fiberglass is sold by the bag at big-box stores and insulation supply houses. Prices vary by tier: contractor-grade bags usually cost less per R than premium homeowner bags, and cellulose is often the cheapest per R when bought in volume, though it settles more over time. Energy code minimums follow climate zone — the IECC calls for R-49 in most cold zones and R-38 in the warm South, while DOE recommends R-49 to R-60 for added insulation in northern attics. Utility and Energy Star rebates in cold-climate states like Minnesota, Wisconsin, and across New England can offset material cost meaningfully. Many retailers lend a blower machine free with a minimum purchase, commonly 10 to 20 bags, so your bag count determines whether the rental is included.
Frequently asked questions
How do I measure my attic for this calculator?
Measure the interior floor area of the attic, not the footprint of the whole house. Run a tape measure along the joists for length and width. For irregular or L-shaped attics, break the space into rectangles, calculate each section separately, and add them together. You don't need to subtract joist bays — blown-in material fills around and over the framing automatically.
What R-value should I target for my attic?
The IECC code minimum is R-49 in most cold climate zones and R-38 in the warm South; the DOE recommends R-49 to R-60 for attics in northern zones. If you already have some insulation, you only need to add enough to reach the target. Measure the existing depth first — blown fiberglass is roughly R-2.5 per inch and cellulose about R-3.5 per inch — then subtract that from your goal before running the calculator.
Why does coverage per bag drop as R-value increases?
Coverage per bag is tied directly to settled fill depth, and it's printed on the bag label. Reaching R-49 with blown fiberglass takes roughly 18 to 19 inches of depth, while R-30 needs only about 10 to 13 inches. More depth means each bag spreads over less floor area, which is why you can't multiply one coverage figure across every R-value. Always check the coverage chart on the specific bag you're buying.
Can I rent a blower machine and do this myself?
Yes — blown-in attic insulation is one of the more DIY-friendly insulation jobs. Most big-box stores lend the blower free when you buy a minimum number of bags, often 10 to 20. Two people makes it much faster: one feeding bags into the hopper below, one directing the hose in the attic. Wear an N95 or better respirator, eye protection, and work in the cool early hours, since attic temps can top 130°F in summer.