Insulation Calculator
Work out how many packages of batts or rolls, or bags of blown-in insulation, you need for an attic or walls.
How to use this insulation calculator
Choose whether you're insulating an attic floor or walls and enter the size in feet. For walls, add up the length of all the walls you're insulating. Then enter the coverage printed on the product:
- Batts and rolls: the package says how many square feet it covers, for example "40 sq ft". Make sure it matches your stud or joist spacing (16 or 24 inches).
- Blown-in: the bag has a coverage chart. Find the row for the R-value you want and use its "square feet per bag" number. Deeper insulation means fewer square feet per bag.
What R-value do I need?
| Area | Typical recommendation |
|---|---|
| Attic, warm climates | R-30 to R-49 |
| Attic, cold climates | R-49 to R-60 |
| 2×4 walls | R-13 to R-15 |
| 2×6 walls | R-19 to R-21 |
These are general ranges. The US Department of Energy's Energy Saver site has a recommendation map by zip code, and your local building code may set a minimum.
The math behind it
Area = length × width for an attic, or wall length × wall height for walls. Packages = area × (1 + extra%) ÷ square feet per package, rounded up. The calculator doesn't subtract windows or doors from walls, so the extra goes toward cuts and odd-sized spaces.
Example: a 40 × 30 ft attic is 1,200 sq ft. With 10% extra and bags that cover 40 sq ft at your R-value, that's 33 bags.
Often used together
Quick answers
How many bags of blown-in insulation do I need?
Divide your attic area by the square feet per bag listed on the bag's chart for your target R-value. A 1,000 sq ft attic at around 40 sq ft per bag needs 25 bags, plus a little extra.
Can I put new insulation on top of old?
Usually yes, as long as the old insulation is dry and not moldy. Don't put faced batts (with paper) on top of existing insulation; use unfaced batts or loose fill instead.
Should I use batts or blown-in in the attic?
Blown-in fills gaps around wires and odd framing better and is fast with a rented machine. Batts are easier to handle without equipment, especially in small or open spaces.