Insulation Calculator
Enter the area you're insulating, the R-value you're aiming for and the material. The calculator returns the R-value to add over any existing insulation, the installed thickness, and the quantity in the unit you actually buy — bags for loose-fill, batts for fiberglass, or board-feet and kits for spray foam.
Your measurements
Attics: R-30 to R-49 (R-60 in cold zones). Walls: R-13 to R-21.
Coverage
- Area to insulate
- 1,000 ft²
- R-value to add
- R-49
- Installed thickness
- 19.6 in
Bags needed
36 bags
Blown-in fiberglass · 19.6 in deep
Materials
- Bags of loose-fill
- 36 bags
- Coverage per bag
- 28 ft² @ R-49
Estimates only. Verify against your supplier's coverage figures before ordering.
Footprint
Footprint to scale
The formula
Thickness = R-to-add ÷ R-per-inch · Loose-fill bags = Area × R-to-add ÷ coverage constant
Example: A 1,000 ft² attic taken to R-49 with blown-in fiberglass (none existing) needs about 19.6 in and ~36 bags. The same attic in cellulose needs ~14 in and ~46 bags.
How it works
- 1Multiply length × width for the area you're covering (one attic or wall plane at a time).
- 2R-value to add = target R − any existing R-value already in place.
- 3Thickness = R-to-add ÷ the material's R-per-inch (≈2.5 blown fiberglass, 3.5 cellulose, 3.1 batts, 3.7 open-cell foam, 6.5 closed-cell foam).
- 4Loose-fill: a bag's coverage shrinks as the target R rises, so bags ≈ area × R-to-add ÷ a coverage constant from the bag chart.
- 5Batts are counted per 16-inch-on-center bay; spray foam is sized in board-feet (ft² × inches) and pro kits of ~600 board-feet.
Frequently asked questions
How much insulation do I need in my attic?
The U.S. Department of Energy recommends roughly R-30 to R-49 for most attics, and up to R-60 in the coldest climate zones. Enter your target R-value and the calculator returns the depth and the number of bags or batts to reach it.
How many bags of blown-in insulation will I need?
It depends on the area and the R-value, because each bag covers less area the deeper you blow it. As a rule, a bag of blown fiberglass covers about 46 ft² at R-30 and a bag of cellulose about 36 ft² — the calculator scales this to your exact target.
Can I add insulation over what's already there?
Yes — for attics you can blow new loose-fill or lay unfaced batts over existing insulation. Set the existing R-value in the advanced options and the calculator only sizes the R-value (and material) you still need to add.
Is the bag count exact?
Treat it as a close estimate. Coverage varies by brand and by how the material settles, and R-per-inch values are mid-range. Always check the coverage chart printed on the bag and add a little for settling.