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Walls & DrywallInsulation Calculator — How Much Insulation Do I Need?
Enter the length and height of the wall or ceiling area to insulate to get the square footage to buy, plus a 5% allowance. Match that figure to the coverage printed on each bag of batts.
How this calculator works
We multiply the area (length × height) and add 5% for trimming and fit. Insulation bags are labeled by the square feet they cover at a given R-value, so divide this figure by your bag’s coverage to count bags.
What changes how much you need
- Bag coverage drops as R-value rises — an R-13 bag covers far more area than an R-30 bag of the same size.
- For walls, match the batt width to your stud spacing (16" or 24" on center).
- Don’t compress batts to fit — compressed insulation loses R-value.
- Subtract large openings (windows, doors, garage doors) for a tighter estimate.
Frequently asked questions
How much insulation do I need?
Measure the wall or ceiling area (length × height) and add about 5%. Then divide by the square-foot coverage printed on each bag of batts, which depends on the R-value.
How many square feet does a bag of insulation cover?
It varies by R-value — a bag of R-13 might cover ~100 sq ft while the same-size R-30 bag covers far less. Always read the bag’s labeled coverage.
What R-value do I need?
It depends on climate and location (walls vs attic). Attics often call for R-38 to R-60; check your local code or the DOE recommendations for your zone.