Insulation Calculator — Batts & Bags
Estimate batt or roll insulation for walls or ceilings. Enter the area and your framing spacing, and this calculator gives the net coverage needed and the number of bags.
How to use this calculator
- Enter the gross wall or ceiling area you are insulating.
- Pick your framing spacing — studs and joists take up cavity space, so the net insulated area is a bit less.
- Enter the coverage per bag from the product label; it varies by R-value and batt size.
- Match the batt width to your framing (15 in batts for 16 in on-center studs, 23 in for 24 in).
The formula
Insulation fills the cavities between framing, so the net area is slightly less than the gross.
Net cavity area = gross area × framing factor (≈0.90 for 16 in on center, ≈0.94 for 24 in).
Bags = net area ÷ coverage per bag, rounded up. Coverage is on the package and depends on R-value and batt size.
Worked example
500 sq ft of wall, 16 in on-center studs, R-13 batts (≈88 sq ft/bag).
Net area = 500 × 0.90 = 450 sq ftBags = 450 ÷ 88 = 5.1 → 6 bags
A 500 sq ft wall at 16 in on center needs about 6 bags of R-13 batts.
Frequently asked questions
How many bags of insulation do I need?
Multiply the area by the framing factor (≈0.90 for 16 in on center), then divide by the coverage printed on the bag. A 500 sq ft wall takes roughly 6 bags of R-13.
What R-value do I need?
It depends on climate and location. Common targets are R-13 to R-21 for 2×4/2×6 walls and R-38 to R-60 for attics. Check your local energy code for required values.
What batt width for my studs?
Use 15 inch wide batts for framing 16 inches on center and 23 inch batts for 24 inches on center, so the batts friction-fit the cavities without gaps or compression.