How it works
The calculator finds the wall area, subtracts doors and windows, adds any extra allowance and divides by the coverage of one bag.
Gross area = Length × Height
Area to insulate = Gross area − Σ (Width × Height × Count of openings)
Bags = ⌈ Area to insulate × (1 + Extra ÷ 100) ÷ Coverage per bag ⌉
⌈ ⌉ means rounding up to the next whole bag. Rounding happens only at the end.
There is no default coverage because it changes with the product, R-value, thickness and stud spacing. Johns Manville’s fact sheet shows how much it varies between products; use the number on your bag. Units are converted internally, so you can mix feet and inches, or enter metric sizes with coverage in square meters.
Worked example
Inputs
- Measure by
- Wall length × height
- Wall length
- 40 ft
- Wall height
- 8 ft
- Doors & windows (not insulated) 1
- Width 3 ft, Height 4 ft, Count 2
- Coverage per bag
- 106.56 sq ft
- Extra allowance
- 0 %
Result
- Gross area
- 320 sq ft
- Doors & windows
- 24 sq ft
- Area to insulate
- 296 sq ft
- Bags
- 3
Common mistakes
- Using the coverage of a different product. Bags that look alike can cover very different areas depending on R-value, thickness and whether the batts are 15 or 23 in wide – read the bag you are buying.
- Choosing an R-value from a rule of thumb. The required value depends on your climate zone and local energy code; ask your building department.
- Forgetting to subtract large openings – or subtracting small ones such as outlet boxes, which are insulated around rather than skipped.
- Mixing up wall area and floor area for an attic. For an attic floor, enter the floor area, not the area of the roof slopes.
Assumptions and limits
- In dimension mode the walls form one rectangle of total length × height; doors and windows are rectangles of width × height × count.
- The bag coverage you enter already reflects the batt width and stud spacing the product is made for. Framing members are not subtracted separately.
- Bags are rounded up to whole bags after the extra allowance is added. Without a coverage figure, only the area is shown.
- This is a material quantity only. It does not choose an R-value, a vapor retarder or an air barrier – those follow your local code.
Frequently asked questions
How many square feet does a bag of insulation cover?
It depends on the product. Johns Manville’s fact sheet, for example, lists 106.56 sq ft per bag for R-13 batts sized 15 × 93 in; other R-values, widths and brands cover different areas. The figure is printed on every bag.
What R-value do I need?
That is set by the energy code where you build, and it differs by climate zone and by part of the house. This calculator doesn’t suggest a value – check with your local building department.
Should I subtract the studs?
No. Batts are made to fit between framing at a given spacing, and the coverage on the bag is the wall area they insulate, studs included. Just enter the wall area minus doors and windows.
Can I use this for an attic floor?
Yes. Switch to “known area”, enter the floor area between the joists you will cover, and use the coverage from the bag of the product you plan to lay.
Sources
- Fiberglass Building Insulation Fact Sheet — Johns Manville. Coverage per bag differs by product, e.g. R-13 15 × 93 in batts: 106.56 sq ft per bag.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 2, 2026