How this calculator works
It multiplies the total wall length by the wall height, takes off doors and windows, adds a small waste allowance and divides by the coverage printed on the bag. The result is rounded up to whole bags.
Coverage is not subtracted for the studs themselves, so the count leans slightly high. Bag coverage differs by product, thickness and pack size, which is why it is a field: enter the number on the bag you plan to buy.
wall area = total wall length × height − 21 ft² per door − 15 ft² per window
bags = wall area × (1 + waste) ÷ coverage per bag
vapor retarder rolls = wall area × 1.10 ÷ roll areaWorked example
Insulating 40 ft of 8′ wall gives 320 sq ft. Take off one door (21) and three windows (45) and 254 sq ft is left. With 5% waste that is 266.7 sq ft; at 40 sq ft per bag it is 6.67 bags, so the list buys 7 bags.
Tips before you buy
- Buy batts the width of your stud bays: 15″ for 16″ framing and 23″ for 24″ framing. They are sized to friction-fit between studs.
- Do not compress batts to fit a shallow cavity; squashed insulation loses part of its rated R-value.
- Split batts around wiring and pipes instead of stuffing them behind; gaps and voids cut performance.
- Check local code before adding a vapor retarder. In mixed and hot climates a poly sheet can trap moisture in the wall.
Questions
- How much does a bag of insulation cover?
- It depends on the product. Bags of R-13 batts for 2×4 walls commonly cover around 40 sq ft, while thicker R-19 and R-21 bags cover less per bag. The coverage is printed on the bag, so enter that number for an exact count.
- What R-value do I need for walls?
- 2×4 walls usually take R-13 or R-15 batts and 2×6 walls R-19 to R-21. The minimum depends on your climate zone, so check the energy code your local building department uses.
- Do I subtract windows and doors?
- Yes, for anything larger than a small window. This calculator takes off 21 sq ft per door and 15 sq ft per window. The off-cuts from around openings go into the narrow spaces above and below them.
- Can I use this for an attic or floor?
- Yes. Enter the length and width of the area in place of wall length and height, set doors and windows to zero, and use the coverage from the bag of the thicker product you plan to buy.