Insulation calculator
Insulation is sold by the bag, and every bag covers a different number of square feet depending on R-value and thickness. Enter the space, pick a product and R-value, and the calculator converts the area into bags, with the installed depth for blown-in and the net area after doors and windows for walls.
How the insulation calculator works
For walls, the calculator multiplies total wall length by height, subtracts about 20 sq ft per door and 15 per window, then takes 90 percent of what is left because studs, plates, and headers occupy the rest. For attics and floors it uses the area directly. It then divides by the coverage per bag, which is printed on every bag label and varies widely: an R-13 bag covers more than 100 sq ft while an R-38 bag covers about 43.
Blown-in products are figured in bags per 1,000 sq ft at the target R-value, using typical manufacturer charts for cellulose (about R-3.7 per inch) and blown fiberglass (about R-2.7 per inch). The depth shown is what you should see on the attic rulers after installation, before any settling.
Formula
wall cavity area = (wall length × height − 20 × doors − 15 × windows) × 0.9
bags = area × 1.05 ÷ coverage per bag
Example: a house with 120 ft of exterior wall at 8 ft, 2 doors and 8 windows, has 800 sq ft of gross wall and about 720 sq ft of cavity. With R-13 batts at 106 sq ft per bag that is 8 bags. Blowing a 1,200 sq ft attic to R-38 with cellulose takes about 42 bags at roughly 10.3 in deep.
Recommended R-values
| Location | Warm climates (zones 1–2) | Mixed (zones 3–4) | Cold (zones 5–8) |
|---|---|---|---|
| Attic | R-30 to R-49 | R-38 to R-60 | R-49 to R-60 |
| 2×4 walls | R-13 | R-13 to R-15 | R-15 plus exterior foam |
| 2×6 walls | R-19 | R-19 to R-21 | R-21 |
| Floors over unheated space | R-13 | R-19 to R-25 | R-30 |
Figures follow the Department of Energy guidance. Air sealing before insulating matters as much as the R-value; caulk and foam the top plates, penetrations, and attic hatch first. Finishing the walls afterward? The drywall calculator picks up where this leaves off.
Common sizes, already worked out
Common questions
How many bags of insulation do I need for 1,000 square feet?
For an attic blown to R-38, about 33 bags of cellulose or 31 of blown fiberglass. For walls with R-13 batts, about 10 bags, since each covers 106 sq ft of cavity. Check the coverage on the specific bag.
How much does a bag of insulation cover?
Batts: roughly 106 sq ft for R-13, 87 for R-19, 58 for R-30, and 43 for R-38. Blown cellulose at R-38 covers about 30 sq ft per bag. The coverage chart is printed on every bag.
Is blown-in or batt insulation better for an attic?
Blown-in fills around wiring, pipes, and odd framing with no gaps, which makes it perform better in a typical attic. Batts are easier for a clean, open attic you may want to walk in later and for floors and walls.
How deep should attic insulation be?
About 10 to 14 in for R-38, depending on material, and 13 to 18 in for R-49. If you can see the top of the ceiling joists, you almost certainly need more.