PreConstruct AIFree calculator
Batt insulation calculator — bags for walls and ceilings
Pick the batt and stud, type the wall length and height, take off the openings, and get the bags — at the coverage printed on the bag, which you can change for any brand.
Bag
21 bags, 1,140 SF.
Bags
21
59.7 SF a bag
Area
1,140 SF
105.91 m²
The working
- walls 140 LF × 9 ft − openings 120 SF = 1,140 SF
- 1,140 SF × waste of 5 per cent ÷ 59.7 SF a bag = 21 bags
A starting figure, not an engineered quantity. Sizes, reinforcing and code requirements come from your drawings, your specifications and your engineer — check them before you order.
Where a calculator stops
This does one run, from numbers you type. A bid is every run on every sheet.
- — It doesn't read the drawing set or its schedules
- — It doesn't measure on the PDF or keep the lengths
- — It doesn't show what an addendum changed
- — It doesn't price, add labour or build a proposal
Area
Wall length × height, less windows and doors, plus any ceiling or floor at the same spacing: LF × ft − openings + ceiling = SF.
Bags
Divide the area by the bag's coverage: SF × waste ÷ SF per bag. The square feet on a Comfortbatt bag is the batts' own face area (12 batts of 47 × 15¼ in is 59.7 SF), and each batt is a little wider than its cavity so it presses in, so a bag fills about 5 per cent more wall than it says. Dividing the gross wall area by the bag figure leaves you that much over. The presets are ROCKWOOL's Canadian bags (its residential product guide, May 2025): R14 for 2×4 at 16 in, 59.7 SF; at 24 in, 60.1 SF; R22 for 2×6 at 16 in, 39.8 SF; R24 for 2×6 at 16 in, 29.4 SF. Fibreglass bags differ — type the number on yours.
How many bags of Comfortbatt for a wall? The coverage of each bag size, for wood and steel studs, and a 40 ft wall worked two ways.
The R-value is not ours to pick
The required R-value comes from the drawings and the energy compliance path for the permit — in Ontario, SB-12 for houses and SB-10 for other buildings. This counts bags for the batt you type.
Questions estimators ask
- How many square feet are in a bag of R14?
- ROCKWOOL Comfortbatt R14 for 2×4 studs at 16 inches is labelled 59.7 SF a bag in Canada. Other brands and thicknesses differ, so check the bag.
- Do I subtract windows and doors from insulation?
- Yes. Take the opening areas off the wall area, then add a small waste allowance for cutting around boxes, blocking and odd bays.
- Does bag coverage allow for the studs?
- Not exactly. Comfortbatt's labelled square feet is the batts' own area; each batt is wider than the space between studs so it presses in, and a bag fills about 5 per cent more wall than it says. Use the gross wall area less openings, not the area between studs, and that 5 per cent is your margin.
- What R-value do I need in Ontario?
- It depends on the building, the assembly and the compliance package on the drawings. For houses see SB-12; for other buildings, SB-10. Use what the drawings specify.
Stop retyping sizes into a calculator
On PreConstruct AI you measure on the PDF, type the sizes the drawings print, and the concrete, forms, bars, board and stringers come out with the arithmetic beside every line — in imperial, metric or both. Your first bid is free, with no card.