Insulation Calculator
Enter the area you want to insulate and pick batts or loose-fill to see how many packages or bags to buy. Package coverage is editable because it changes with the product and R-value.
How to use this insulation calculator
- Measure the length and width of the wall, floor or attic you want to insulate. For several sections, calculate each and add them.
- Subtract any area you will not cover, such as windows.
- Choose batts or rolls, or loose-fill. Batts use coverage per package. Loose-fill uses depth and volume per bag.
- Check the numbers on the product label and change the defaults if they differ.
- Keep or change the waste allowance, add a price per package if you like, and press Calculate.
How to calculate how much insulation you need
For batts and rolls the calculator works by area. For loose-fill it works by volume.
- Area = length (ft) x width (ft) – area to subtract
- Batts: packages = area x (1 + waste % / 100) / coverage per package, rounded up
- Loose-fill: volume (cu ft) = area x depth (in) / 12 x (1 + waste % / 100)
- Bags = volume / volume per bag, rounded up
- Cost = packages or bags x price
Worked example
An attic floor is 20 ft by 10 ft, which is 200 sq ft. For batts with 5% waste you need 200 x 1.05 = 210 sq ft. A package covers 40 sq ft, so 210 / 40 = 5.25, which rounds up to 6 packages. At $55 per package the batts cost 6 x 55 = $330.00.
For loose-fill at 10 inches deep, the volume is 200 x 10 / 12 = 166.7 cubic feet, and with 5% waste it is 175 cubic feet. With 20 cubic foot bags that is 8.75, so 9 bags.
Assumptions & limits
- The default package coverage of 40 sq ft and bag volume of 20 cubic feet are editable placeholders. They change with the product, the R-value and the wall depth.
- The calculator does not choose an R-value. Recommended R-values depend on climate zone and the part of the building, so check your local energy code or the product guide.
- Loose-fill settles over time, so the final depth may be less than the installed depth.
- Batts need to fit the framing spacing. Framing, wiring and plumbing reduce the area that insulation can cover.
- The price is your own input. Vapor barriers, air sealing and labor are not included.
Frequently asked questions
How do I calculate how much insulation I need?
Find the area in square feet, subtract any openings, add a waste allowance and divide by the coverage per package. A 200 sq ft attic with 5 percent waste and 40 sq ft packages needs 6 packages.
How many bags of blown-in insulation do I need?
Multiply the area by the depth in feet to get cubic feet, add waste, and divide by the bag volume. The product label usually has a chart of bags per square foot for each R-value.
What R-value do I need?
It depends on your climate, the part of the building and local code. Check the local energy code or the product guide. This calculator counts packages and does not recommend an R-value.
Does insulation coverage depend on thickness?
Yes. A thicker batt covers less area per package than a thinner one, so always use the coverage printed for the product you buy.
Should I add waste to the order?
A small allowance covers cuts and trimming. The calculator starts at 5 percent and you can change it.
Can I use this for walls and attics?
Yes. Enter the length and width of the area. For a wall, use the length and height.
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Last updated: November 2026