How this insulation calculator works
Insulation is sold by the bag and estimated by area. The calculator takes the area you are insulating, adds a waste and settling allowance, and divides by the coverage of one bag — the same honest area-to-packages method the drywall and flooring calculators use:
purchase basis = area × (1 + waste %)
bags = round up (purchase basis ÷ coverage per bag)
Blown-in: coverage depends on R-value
For loose-fill, a single bag covers a large area at a low R-value and a small area at a high one, because deeper insulation uses more material per square foot. Every bag has a coverage chart listing the square feet it covers at each R-value — use the figure for the R-value you need.
| Target R-value | Typical coverage per bag | Common use |
|---|---|---|
| R-30 | ~60 sq ft | Warmer climates, floors |
| R-38 | ~48 sq ft | Attic floors (common) |
| R-49 | ~37 sq ft | Cold climates, attics |
Batts and rolls
Pre-cut batts come sized for 15-inch or 23-inch stud and joist bays. Each bag covers a roughly fixed area printed on the pack. Match the batt width to your framing spacing and the R-value to your assembly.
What this does not do
It does not tell you what R-value to use, specify a vapor barrier or ventilation, or certify an assembly — those are set by your building code and climate, and are outside a material-quantity tool. Confirm the required R-value locally, and read the coverage off your own bag before buying.
How to estimate insulation
- Pick blown-in or batt, and enter the area (or room/attic size) you are insulating.
- Choose your product, or enter the coverage from your bag — for blown-in, the coverage at your target R-value.
- Set a waste/settling allowance (5–10% is typical).
- Read the number of bags to buy, with the purchased area and leftover shown.
- Confirm the R-value your code requires before buying — this tool sizes the quantity, not the assembly.
Frequently asked questions
How many bags of insulation do I need?
Take the area you are insulating, add a waste/settling allowance, and divide by the coverage on your bag — then round up. For blown-in, the coverage per bag depends on your target R-value (deeper = fewer sq ft per bag), and it is printed on the bag’s chart. A 1,000 sq ft attic at R-38, where a bag covers about 48 sq ft, needs about 23 bags with 10% allowance. Enter your numbers above.
Does this tell me what R-value I need?
No — and it should not. The required R-value is set by your local building code and climate zone, not by a calculator. This tool works out how many bags or rolls to buy once you know your target R-value and your product’s coverage. Confirm the R-value with your local code or a pro.
How is blown-in different from batts?
Batts and rolls come pre-cut to fit stud and joist bays, and each bag covers a roughly fixed area. Blown-in (loose-fill cellulose or fiberglass) is poured or blown to a depth, so one bag covers more area at a low R-value and less at a high one — always read the coverage chart on the bag for your R-value.
Should I add extra for settling?
Yes for blown-in — it settles over time, so installers add material above the target depth. A 5–10% allowance is a reasonable planning figure; follow the bag’s stated coverage, which usually already accounts for settled thickness.
Can I use this for walls as well as attics?
Yes. Enter the net wall or ceiling area (minus openings) for batts, or the attic floor area for blown-in. For wall board quantities see the drywall calculator.