Insulation Calculator
A 15 by 20 ft attic (300 sq ft) needs 8 bags of insulation at 40 sq ft per bag with 5% waste. Enter the length and width of the space, the coverage printed on the insulation bag, and a small waste allowance to get the number of bags to buy. The calculator divides your area by the coverage per bag and rounds up so the whole space is covered.
Quick answer
Bag count is the area divided by the coverage per bag, rounded up to whole bags.
What this tells you
- •Bag count is the area divided by the coverage per bag, rounded up to whole bags.
- •Coverage per bag depends on the R-value you choose, so read the number on the bag.
- •A small waste allowance covers settling, gaps, and uneven depth.
How to Use
- 1Enter the length of the space in feet.
- 2Enter the width of the space in feet.
- 3Enter the coverage per bag from the bag label (40 sq ft is a common value).
- 4Set a waste percentage (5% works for most jobs).
- 5Click Calculate to see how many bags or rolls you need.
How It Works
Formula
Area = Length x Width
Bags = ceil(Area x (1 + Waste / 100) / Coverage per Bag)The area is the length times the width of the space. Divide that area by the coverage per bag listed on the label, add the waste allowance, and round up so the entire area is covered. Coverage per bag changes with the R-value you select, so use the value printed on the product you buy.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
15 by 20 ft attic at 40 sq ft per bag
Area = 15 x 20 = 300 sq ft. Bags = ceil(300 x 1.05 / 40) = ceil(7.88) = 8 bags.
20 by 25 ft area at 40 sq ft per bag
Area = 20 x 25 = 500 sq ft. Bags = ceil(500 x 1.05 / 40) = ceil(13.13) = 14 bags.
Insulation Bags by Area
Bag counts for common areas at 40 sq ft per bag with 5% waste added. Adjust the bag count if your chosen R-value changes the coverage.
| Area (sq ft) | Bags Needed |
|---|---|
| 200 | 6 |
| 300 | 8 |
| 500 | 14 |
| 800 | 21 |
| 1000 | 27 |
Coverage of 40 sq ft per bag is an example value. Read the coverage chart on your bag for the R-value you plan to install.
Reading Coverage on an Insulation Bag
Every bag of blown-in insulation prints a coverage chart that lists how many square feet one bag covers at each R-value. Higher R-values mean a deeper layer, so one bag covers fewer square feet. Always read the bag for the R-value you want before counting bags.
Batts and rolls list coverage on the package too, usually as the total square feet per package. Match the batt width to your joist or stud spacing (16 or 24 inches on center) so the pieces fit without gaps.
Because coverage shifts with R-value and product type, treat 40 sq ft per bag as a starting example. Plug in the actual coverage from the bag you buy to get an accurate count for your project.
Common mistakes
- Using a generic coverage number instead of the value for your target R-value
- Forgetting that higher R-values mean fewer square feet covered per bag
- Measuring the floor when the insulation goes on a sloped or larger surface
- Skipping a waste allowance for settling and uneven coverage