Insulation Calculator
Enter the area to insulate and the coverage per bag or roll to estimate how many packages you need.
Results
- Packages needed
- 14
- Area
- 500 sq ft
- Area incl. waste
- 550 sq ft
Estimates only. Round up and buy a little extra to account for waste, spills, and breakage.
This calculator sizes insulation by area and reports the bags or batt bundles needed to hit your target R value.
Coverage per bag falls as depth rises, so a bag of blown cellulose that covers 40 sq ft at R 30 will only cover about 24 sq ft at R 49.
Also works as a batt insulation calculator, R value calculator, blown in insulation calculator, and attic insulation estimator.
How to use the insulation calculator
- Enter the area to insulate in sq ft.
- Enter the coverage per package in sq ft.
- Enter the waste factor in %.
- Optionally add a price to estimate the total project cost.
- Read your quantities instantly — no sign-up, and you can print the estimate to take to the supplier.
Worked example
Inputs
- Area to insulate
- 500 sq ft
- Coverage per package
- 40 sq ft
- Waste factor
- 10 %
Results
- Packages needed
- 14
- Area
- 500 sq ft
- Area incl. waste
- 550 sq ft
Using the example values above, the insulation calculator returns 14 (packages needed). Change any field to match your own project and the numbers update instantly.
Recommended R values by zone
Attics: R 30 to R 49 in warm southern zones, R 49 to R 60 in cold northern zones. Walls: R 13 to R 21. Floors over unheated space: R 19 to R 30.
Depth guide for blown fibreglass: about 14 inches for R 38 and 18 inches for R 49. Cellulose is denser and needs roughly 11 and 14 inches for the same values.
Batts versus blown in
Batts suit open stud and joist bays at standard 16 or 24 inch spacing. Cut them to friction fit with no gaps or compression, since compressed batts lose R value.
Blown insulation is better for attics with obstructions, existing insulation to top up, or irregular joist spacing. Air seal penetrations before adding either.
How much extra should I order?
Always order a little extra. A 5% waste factor suits simple, square projects; 10% is right for most jobs; bump it to 15–20% for irregular shapes, diagonal patterns, or uneven ground. Running short almost always costs more than the leftover material.
Formula
Packages = (area × (1 + waste%)) ÷ coverage per package.
Frequently asked questions
- What R-value do I need?
- Attics often need R-38 to R-60; walls typically R-13 to R-21 depending on climate.
- How much insulation waste should I add?
- Add 5–10% for trimming around studs, wiring, and outlets.
- How many bags of insulation do I need for my attic?
- Divide attic square footage by the coverage at your target R value printed on the bag, then round up. A 1,000 sq ft attic at R 49 typically needs 40 to 45 bags of cellulose.
- How thick is R 30 insulation?
- About 9.5 inches for fibreglass batts, 10 to 11 inches for blown fibreglass and roughly 8 inches for cellulose.
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