How this calculator works
The calculator divides the net area you’re insulating by the square footage that one bag of blown-in insulation or one batt pack covers at your selected R-value. Choose the package coverage that matches both the insulation type and the installed R-value; the same package can cover a different area at a different thickness.
30 × 25 = 750 sq ft. 750 ÷ 40 = 18.75, so round up to 19 bags.
Measure the net area, then round the order up
Measure each attic floor or wall section separately, subtract areas you will not insulate, then add the net square footage. Round up to whole bags or packs and allow a small cushion for cut batts, irregular bays, and settling. For batts, verify the product width matches your stud or joist spacing. For an unfinished wall project, estimate covering material with the drywall calculator.
Frequently asked questions
How do I know how many bags of insulation I need?
Multiply length by width for each section, subtract exclusions, then divide the net area by the package coverage for your target R-value and round up.
Does insulation coverage change with R-value?
Yes. Higher R-values require more thickness, so a bag of blown-in insulation can cover fewer square feet. Use the manufacturer’s coverage chart for the exact R-value and product.
Should I add extra insulation for waste?
Round up to full packages. A small cushion can help with irregular bays, cut batts, and settling, but confirm the product instructions and local requirements before purchasing.
What R-value do I need for an attic?
Requirements vary by climate zone and local code. Check the applicable code and the manufacturer’s installation instructions before selecting a target R-value.