Insulation Calculator
Enter the area you're insulating and the coverage printed on the package. Works for batts, rolls and blown-in insulation — just read the coverage for your target R-value off the bag.
Results
How the insulation calculator works
Insulation is bought by coverage: each bundle of batts, roll or bag of blown-in fibre lists how many square feet it covers at a given R-value. The calculator takes your area, adds a waste allowance for cuts and framing, and divides by that coverage figure — then rounds up to whole packages.
Formula: area × (1 + waste%) ÷ coverage per package = packages (rounded up)
| Location | Typical R-value |
|---|---|
| Attic / ceiling | R-38 to R-60 |
| 2×4 exterior wall | R-13 to R-15 |
| 2×6 exterior wall | R-19 to R-21 |
| Floors over crawlspace | R-19 to R-30 |
The higher the R-value, the fewer square feet a bag covers, because the material is applied thicker. That is why the coverage number — not the bag count — is what you enter here.
Batts, rolls or blown-in?
- Batts and rolls suit open stud and joist bays. Match the batt width to your framing (15" for 16" on-center studs, 23" for 24") and the depth to the cavity.
- Blown-in cellulose or fibreglass is faster for attics and fills odd shapes — rent or borrow a blower and read the bag chart for how many bags per 1,000 sq ft at your target R.
- Don't compress insulation to make it fit — a squashed R-19 batt performs like a lower R-value. Cut it to fit around wiring and boxes instead.
- Leave a ventilation gap at the eaves with baffles so attic airflow isn't blocked.
Frequently asked questions
How many bags of insulation do I need?
Divide the area by the coverage printed on the bag for your target R-value, then add about 5–10% for waste and round up. A 1,200 sq ft attic with bags covering 40 sq ft each needs about 30 bags — 32 with a 5% allowance.
How do I calculate insulation for a wall?
Multiply wall length by height for the area, add about 10% for cuts around studs, windows and boxes, then divide by the coverage of the batt bundle you're buying. Match the batt width to your stud spacing.
How much insulation do I need for a 1500 sq ft attic?
For 1,500 sq ft at R-38, that's about 1,575 sq ft with a 5% allowance. If each bag of blown-in covers 40 sq ft at R-38 you'd need about 40 bags — but always check your bag's coverage chart, since it changes with R-value.
What R-value do I need?
It depends on climate and location: attics commonly call for R-38 to R-60, 2×4 walls R-13 to R-15, and 2×6 walls R-19 to R-21. Check your local energy code for the exact minimum where you live.