BuildRight

Insulation Calculator

Area to cover divided by the coverage each bag or roll provides, per the product label.

Area to insulate (sq ft)

Coverage per bag/roll (sq ft)

Bags/rolls needed

Coverage per unit varies a lot by R-value and product — check the specific bag or roll's label.

What you entered

Area to insulate (sq ft): 500Coverage per bag/roll (sq ft): 40

1
Divide area by coverage per unit
500 ÷ 40= 12.5 units
2
Round up to a whole number of units
⌈12.5⌉= 13 units

Result

Bags/rolls needed: 13

500 sq ft needs 13 bags or rolls at 40 sq ft coverage each.

Insulation units needed by area and coverage

Units are area divided by the coverage each bag or batt package provides, always rounded up. Coverage is printed on the packaging and varies with the R-value and depth you are installing to.

Area to cover	Coverage per unit	Units needed
500 sq ft	40 sq ft	13
1,000 sq ft	25 sq ft	40
1,000 sq ft	40 sq ft	25
1,000 sq ft	88 sq ft	12
1,500 sq ft	40 sq ft	38
2,000 sq ft	88 sq ft	23

Blown-in coverage depends on the depth you are targeting, so the same bag covers far less area at R-49 than at R-30. Check the bag's coverage chart for your target R-value rather than the headline figure, and buy one extra: settling and awkward corners always take more than the arithmetic says.

Coverage depends heavily on R-value

A higher R-value batt or a thicker layer of blown-in insulation covers less area per bag than a lower R-value product, since more material is packed into the same bag — always use the coverage figure from your specific product's label, not a generic estimate.

Attic vs. wall insulation

Attics typically use loose-fill blown insulation measured by bag coverage at a target depth; walls typically use batts measured by square footage of cavity — this calculator's simple area-and-coverage approach works for either, as long as you use the right product's coverage rate.

Frequently asked questions

My attic is 1,200 sq ft and each bag covers 40 sq ft at R-38 — how many bags do I need?

30 bags (1,200 / 40 = 30). The coverage per bag changes significantly with R-value — the same bag that covers 40 sq ft at R-38 might cover 65 sq ft at R-22. Always use the coverage for your target R-value.

What R-value do I need?

It depends on your climate zone and where in the house. Attics: R-38 to R-60 in cold climates, R-30 in mild climates. Walls: R-13 to R-21. Basement/crawlspace: R-10 to R-19. Check the DOE's recommended R-values for your zip code.

Is blown-in or batt insulation better?

Blown-in fills irregular spaces and gaps better, especially in attics with obstructions. Batts are easier to install in standard-dimension wall cavities. For existing walls, blown-in is usually the only option without removing drywall.