Insulation Calculator
Packages = area ÷ the coverage printed on your product's bag or bundle, plus waste; for batts, deduct the framing first. A 30 × 20 ft attic is 600 ft²: with blown-in that covers 38 ft² per bag, that is 15.8 bags, or 17 with 5% extra. Measure walls with the Wall Square Footage Calculator.
How it's calculated: Batts: packages = length × width × (1 − framing %) ÷ coverage per package; blown-in: bags = area ÷ coverage per bag at your target R-value; then add the waste allowance and round up.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Batts: net area = Length × Width × (1 − framing %); packages = net area ÷ coverage per package
Blown-in: bags = area ÷ (coverage per bag at your target R-value)
Run the calculation to see each step with your own numbers.
A 30 ft × 20 ft attic floor, blown-in to R-38 (~38 ft²/bag), 5% over-blow.
Area = 600 ft²
Bags = 600 ÷ 38 = 15.8 → order 17 bags
- Coverage per package/bag comes from YOUR product's bag — it depends on R-value, cavity depth and, for blown-in, the target thickness.
- For blown-in attic insulation, blow to the manufacturer's coverage chart for your target R-value; the bag count assumes you enter that coverage figure.
- Batt framing deduction accounts for the studs/joists batts wrap around — set it to 0 for a quick full-area estimate.
- Vapour retarder, baffles and air-sealing are separate.
- How many bags of blown-in insulation for my attic?
- It depends entirely on your target R-value: an R-38 attic needs roughly a bag per 35–40 ft², R-49 a bag per ~28 ft². Read the coverage chart on the bag for your R-value and enter that number.
- Why do I have to enter the coverage myself?
- Because it isn't fixed. A bag of blown-in insulation covers far more area at R-19 than at R-38, and batt bundle sizes vary by manufacturer. The number printed on your bag is the reliable input.
- What is the framing deduction for?
- Studs and joists take up 10–15% of a wall or floor area, and batts don't fill solid wood. Deducting that gives a truer batt count. Leave it at 0 for blown-in or a quick full-area estimate.
- How do I calculate how many insulation batts I need?
- Multiply length by width, deduct the framing share if you want a net figure, and divide by the coverage on the bundle. A 20 × 10 ft wall is 200 ft²; with 88 ft² bundles that is 2.3, so order 3 with 5% waste. With a 10% framing deduction it is 180 ft² — still 3 bundles.
Coverage
Insulation-manufacturer coverage charts (Owens Corning, Johns Manville, CertainTeed). Coverage is R-value- and cavity-specific — enter your product's figure.
About the Insulation Calculator
The Insulation Calculator converts a wall, floor or attic area into packages of batt or roll insulation, or bags of blown-in, using the coverage figure from your specific product — which depends on the R-value and cavity depth. For batts you can deduct the framing area the batts wrap around; for blown-in you enter the coverage per bag at your target R-value from the chart on the bag.
What you enter
- Length
- Width
- Insulation type
- Coverage per package / bag (ft²) — From the bag. A batt bundle is often 40–110 ft²; a blown-in bag at R-38 covers ~35–40 ft², at R-19 ~65 ft².
- Deduct for framing (batts only) — Studs/joists take up ~10–15% of a wall/floor area that batts don't fill. Leave 0 for blown-in or a full-coverage estimate.
- Waste allowance — Trimming batts to length; over-filling for blown-in settling.
What you get back
- Area — the exact result of the calculation.
- Packages / bags — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Drywall Calculator — Drywall sheets, screws, mud and tape for a room.
- Wall Square Footage Calculator — Wall area for one or more walls, with openings deducted.
- Framing Calculator — Stud count and lineal feet for a wall at a given stud spacing.
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- How to Choose the Right Air Conditioner Size for a Room — Air conditioner sizing starts with room area and a BTU-per-square-foot estimate, adjusted for factors like ceiling height, sun exposure and occupancy.
Part of the Drywall & Insulation calculators.