Insulation Calculator
Enter the wall or ceiling area you need to insulate and the coverage printed on the insulation bag to find how many bags or batts to buy. Coverage per bag depends on the R-value, so use the figure on the label.
How to calculate insulation
Find the area you are insulating in square feet (length × height for a wall, length × width for a ceiling or floor), then divide by the coverage printed on the insulation bag. Coverage falls as the R-value rises, because thicker, higher-R batts mean fewer pieces per bag: a bag of R-13 covers about 106 ft², R-19 about 75 ft², and R-30 about 58 ft². Round up to whole bags and add about 10% for trimming around studs, outlets, and obstructions.
Bags = Area ft² ÷ Coverage per bag (coverage depends on R-value)
Worked example
Insulating 40 linear feet of 8 ft-high 2x4 wall (320 ft²) with R-13 batts that cover 106 ft² per bag.
- Area = 40 × 8 = 320 ft²
- Bags = 320 ÷ 106 = 3.02 → 4 bags
- With 10% waste = 4 bags
Result: 320 ft² of wall needs 4 bags of R-13 batt insulation.
Frequently asked questions
How much insulation do I need?
Work out the area to insulate in square feet (length × height for walls), then divide by the coverage printed on the bag and round up. A 320 ft² wall insulated with R-13 batts that cover 106 ft² per bag needs about 4 bags. Add 10% for trimming and waste.
How many square feet does a bag of insulation cover?
It depends on the R-value, because higher-R batts are thicker and there are fewer per bag. As a rough guide, a bag of R-13 covers about 106 ft², R-19 about 75 ft², and R-30 about 58 ft². Always use the coverage figure on the specific bag, as it varies by brand and batt width.
What R-value do I need?
It depends on where you live and what you are insulating. The US Department of Energy recommends roughly R-13 to R-21 for exterior walls and R-30 to R-60 for attics, with colder climate zones at the higher end. Check the DOE zone map for your area before buying.
Should I subtract windows and doors?
For a tighter estimate, subtract large openings such as windows and doors from the wall area, since you do not insulate them. The 10% waste allowance usually absorbs small gaps, but on a wall with many openings, subtracting them avoids buying extra bags.
Does this work for blown-in or batt insulation?
It works for any insulation sold by area coverage, including batts and rolls. For blown-in insulation, bags are rated for the square footage they cover at a target R-value (or depth), so enter that coverage figure from the bag to get the number of bags.