Cellulose Insulation Calculator
Enter your measurements
Results
- Bags needed42 bags
- Attic area1,200 sq ft
- Coverage per bag28.89 sq ft at R-38
- Fill depth10.9 in
Estimated cost
per sq ft (material)
Material cost per sq ft reflects bag purchase at R-38 depth (about 28.89 sq ft per 25 lb bag) from big-box or supply-house retailers in 2026; installed cost includes contractor labor, blower equipment, and basic air sealing prep. The high end of both ranges reflects dense metro markets, premium recycled-content brands, or jobs needing significant attic prep.
Estimate only — prices vary by region, supplier, and season. Get a local quote before buying.
The cellulose insulation calculator tells you how many bags to buy for your attic based on the square footage you need to cover and your target R-value. Because coverage per bag drops as you add depth — roughly 29 sq ft per bag at R-38 but far more at R-19 — the math is not linear, and getting it wrong means either a second trip to the store or a costly over-purchase. The calculator returns the bag count, the fill depth in inches, and the per-bag coverage so you can cross-check against the coverage chart printed on every bag. Cellulose ships in bags that typically weigh about 25 lbs each and load into a blower machine, which most home centers rent free with a minimum purchase of 10 to 20 bags. Plan for a 10 to 15 percent overage to cover material lost around the blower hose and bags that come up short. The calculator's fill depth already reflects ASTM-rated settled thickness, so you do not add a separate settling buffer on top of it.
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 by 30 ft attic — 1,200 sq ft total — targeting R-38, the calculator returns 42 bags, each covering 28.89 sq ft, blown to a settled fill depth of 10.9 inches. That bag count is what you hand to the cashier or enter into a delivery order; the fill depth tells you where to set your depth indicators before you start blowing so you hit R-38 consistently across the whole attic floor.
Inputs
- Attic length
- 40 ft
- Attic width
- 30 ft
- Target R-value
- R-38
Result
- Bags needed
- 42 bags
- Attic area
- 1,200 sq ft
- Coverage per bag
- 28.89 sq ft at R-38
- Fill depth
- 10.9 in
- Estimated material cost
- $720 – $1,560
Materials & pricing near you
Cellulose is sold by the bag at big-box stores and insulation supply houses, with bag prices generally running about $18 to $35 depending on brand, density rating, and retailer; bulk and pallet pricing typically lands at the low end. Supply houses often require a minimum delivery of a half or full pallet, roughly 25 to 50 bags. Material cost at R-38 depth works out to about $0.60 to $1.30 per sq ft, with rural areas carrying higher freight sitting at the upper end. Installed pricing through a local insulation contractor typically falls in the $1.30 to $2.80 per sq ft range, covering labor, blower time, and basic air sealing. A simple attic top-off rarely needs a building permit, but check your local building department before any job that touches wiring or adds attic floor structure.
Frequently asked questions
How deep do I blow the cellulose to hit my target R-value?
The calculator returns the fill depth automatically once you enter your attic area and target R-value; for R-38 that works out to roughly 10.9 inches of settled cellulose. Before you start, staple depth rulers or push wire flags into the joists every few feet so you can confirm depth as you go. Per the FTC R-Value Rule, the figure on the bag's coverage chart and in this calculator is the settled depth, not the fluffed initial depth, so blow to those markers and let it settle over the next day or two.
Does cellulose settle, and do I need extra bags for it?
Cellulose does settle, but the coverage ratings printed on each bag and used in this calculator already reflect settled depth under ASTM C739 and the FTC R-Value Rule, so you do not add a separate settling buffer on top of the bag count. What you should add is a 10 to 15 percent overage for material lost during blowing, stray fill around joists and obstructions, and spots that need touching up.
What R-value does my attic need to meet code?
The 2021 IECC requires R-60 in climate zones 4 through 8 (most of the Midwest, Northeast, Mountain West, and Mid-Atlantic), R-49 in zones 2 and 3, and R-30 in zone 1. R-38, the value in the example, was a common minimum under older codes and is still a frequent retrofit target, but it no longer meets the 2021 IECC in any zone. Many states are still on a 2018 or earlier edition, so confirm the adopted version and minimum with your local building department.
Can I use this calculator for wall cavities or just attics?
Use it for open-blow attics only. Blowing cellulose into walls is a different method — dense-pack into enclosed cavities at roughly 3.0 to 3.5 lb per cubic foot, versus about 1.5 lb per cubic foot for loose-fill attic blowing — so the coverage rates differ sharply. The bag counts and fill depths here are calibrated for open attic floors; for dense-pack walls you need a separate calculation based on cavity volume and the higher install density.