Insulation Calculator
Choose where it goes and the type; set the R-value and bag coverage below.
Walls & Paint
Insulation Calculator
Quick answer: batt packages = area ÷ ft² per package; blown-in bags = area ÷ 1,000 × bags per 1,000 ft² from the bag chart. A 30 × 40 ft attic (1,200 ft²) at R-49, with a chart rate of 51 bags per 1,000 ft², needs 61.2 bags — 65 bags with 5% extra.
Estimate insulation for an attic, floor, or walls: packages of fiberglass batts or rolls, or bags of blown-in cellulose or fiberglass, using the R-value you need and the coverage printed on the product.
How to calculate insulation
Every insulation product states its coverage on the package. Batts and rolls list the square feet in each bag for a given R-value and framing width. Blown-in products print a coverage chart that gives, for each target R-value, the number of bags per 1,000 ft² and the minimum settled thickness. Your job is to measure the area accurately and read the right line of the chart.
Batts: Area ÷ ft² per package
Blown-in: Area ÷ 1,000 × bags per 1,000 ft² — add waste, round up
- Measure the area. Attic or floor: length × width of the space between the exterior walls. Walls: total wall length × wall height.
- Subtract large openings such as windows and doors in walls. Do not subtract studs or joists — package coverage already assumes standard framing.
- Pick the R-value for the location (see the DOE table below for attics, or your local energy code).
- Divide by the coverage from the package or chart, add about 5% for trimming and uneven areas, and round up to whole bags.
Worked example: blowing an attic to R-49
A 30 × 40 ft attic floor is 1,200 ft². The cellulose bag chart in this example lists 51 bags per 1,000 ft² for R-49.
- Bags: 1,200 ÷ 1,000 × 51 = 61.2 bags
- With 5% extra: 61.2 × 1.05 = 64.26 → 65 bags
- Depth check at R-3.5 per inch: 49 ÷ 3.5 = 14 in settled
- At $15 a bag: 65 × $15 = $975 in material
Worked example: R-13 batts in 2×4 walls
120 ft of exterior wall, 8 ft high, with 150 ft² of windows and doors, using packages that cover 40 ft² each.
- Net area: 120 × 8 − 150 = 810 ft²
- With 5% extra: 850.5 ft² ÷ 40 = 21.26 → 22 packages
Recommended attic R-values by climate zone
Ranges for an uninsulated attic, per U.S. DOE / ENERGY STAR guidance. Find your zone on the IECC climate-zone map for your county; local energy codes may require more.
| Climate zone | Example cities | Uninsulated attic |
|---|---|---|
| 1 | Miami, Honolulu | R-30 to R-49 |
| 2 | Houston, Orlando, Phoenix | R-30 to R-60 |
| 3 | Atlanta, Dallas, Los Angeles | R-30 to R-60 |
| 4 | Washington DC, St. Louis, Seattle | R-38 to R-60 |
| 5 | Chicago, Denver, Boston | R-49 to R-60 |
| 6 | Minneapolis, Burlington VT | R-49 to R-60 |
| 7 | Duluth MN | R-49 to R-60 |
| 8 | Fairbanks AK | R-49 to R-60 |
Batts and rolls vs. blown-in
Batts and rolls are sized to fit between framing: about 15 in wide for 16 in on-center studs and joists, and about 23 in wide for 24 in on-center. They suit open wall cavities, floors over crawlspaces, and attics with regular joists and little blocking. A 2×4 wall cavity (3½ in deep) takes R-13 or high-density R-15; a 2×6 cavity (5½ in) takes R-19 or R-21. Do not compress batts into a shallow cavity — squeezing loses part of the rated R-value.
Blown-in cellulose or loose-fill fiberglass fills around wiring, bracing, and irregular joists, and it is the usual way to top up an existing attic. The coverage chart is product-specific: fiberglass bags are lighter and fluffier than cellulose, so the bag count and the depth for a given R-value differ a lot between the two. Use the R-per-inch field to estimate depth — roughly R-3.2 to R-3.8 per inch for cellulose and lower for loose-fill fiberglass — but follow the minimum settled thickness on the bag.
Insulation tips and common mistakes
- Air-seal first. Seal top plates, wiring holes, plumbing chases, and the attic hatch before adding insulation; air leaks bypass any R-value.
- Keep soffit vents open. Install baffles at the eaves so blown insulation does not block intake ventilation.
- Mind recessed lights and chimneys. Keep insulation clear of non-IC-rated fixtures and maintain the clearance your chimney or flue manufacturer and local code require.
- Mark the depth. Staple depth rulers to joists across the attic so you blow an even layer that meets the chart thickness everywhere.
- Count existing insulation. When topping up, subtract the R-value already there from your target — then read the chart for the R-value you are adding.
- Check the vapor retarder rule. Kraft-faced batts face the heated side in most U.S. climates; never stack a faced batt on top of existing insulation. Confirm with local code.
Insulation calculator FAQ
How much insulation do I need for my attic?
The U.S. Department of Energy recommends R-30 to R-49 for uninsulated attics in the warmest zone, R-30 to R-60 in zones 2 and 3, R-38 to R-60 in zone 4, and R-49 to R-60 in zones 5 through 8. Multiply the attic length by width for the area, then use the product coverage for that R-value. For example, a 1,200 ft² attic with a chart rate of 51 bags per 1,000 ft² at R-49 needs about 61 bags before waste.
How many bags of blown-in insulation do I need?
Divide the attic area by 1,000 and multiply by the bags per 1,000 ft² listed on the bag chart for your target R-value. For a 1,200 ft² attic and a chart value of 51 bags per 1,000 ft², that is 1.2 × 51 = 61.2 bags. Round up and add about 5 percent, which gives 65 bags.
How many square feet does a bag of batt insulation cover?
It depends on the R-value and width: thin R-13 wall batts pack more square feet per bag than thick R-30 or R-38 attic batts, and 23 in wide batts for 24 in framing cover more than 15 in wide batts. Use the exact square footage printed on the package you buy, and divide your area by it.
What R-value insulation do I need for walls?
Standard 2×4 walls hold R-13 or high-density R-15 batts, and 2×6 walls hold R-19 or R-21. Energy codes in colder climates may also require continuous insulation on the outside of the wall sheathing. Check your local energy code for the required wall R-value.
How deep should blown-in insulation be for R-49?
At roughly R-3.2 to R-3.8 per inch, cellulose needs about 13 to 15 inches of settled depth to reach R-49. Loose-fill fiberglass has a lower R-value per inch, roughly R-2.2 to R-2.7, so it needs about 18 to 22 inches. The bag chart lists the exact minimum settled thickness for each R-value.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.