To size attic insulation yourself, run two divisions. First, depth: target R-value divided by the R-per-inch of your material gives the inches you need. Second, bags: attic square footage divided by the coverage per bag (at your target depth, printed on the bag chart) gives the bag count. Both numbers are product-specific. For example, Owens Corning’s AttiCat blown fiberglass lists R-49 at 17 inches and 25 bags per 1,000 sq ft, while Greenfiber’s INS541LD blown cellulose lists 83.5 bags per 1,000 sq ft at R-49, so check your own bag’s chart. This attic insulation calculator page walks the exact formula, then shows worked examples by material so you can check any manufacturer number before you buy.
The math is simple once you separate the two questions most people blur together: how deep the insulation has to be, and how much product that depth takes. They use different inputs. Depth depends on R-value and the material. Bag count depends on your attic area and the coverage printed on the bag.
How much insulation do I need for my attic?
Most U.S. attics need R-30 to R-60 of insulation, depending on climate zone. For existing homes, ENERGY STAR’s Recommended Home Insulation R-Values retrofit table recommends bringing an uninsulated attic floor to R-30 in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. If the attic already has 3 to 4 inches of insulation, the same table recommends adding R-25 in zone 1, R-38 in zones 2 and 3, and R-49 in zones 4 through 8. Pick your zone’s number first, because every other calculation depends on it.
R-value is thermal resistance: higher means the ceiling slows heat flow more. The ENERGY STAR figures are retrofit recommendations for existing wood-framed homes. Code minimums for new construction are a separate question: the 2021 IECC (Table R402.1.3) sets prescriptive ceiling minimums of R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8, per the DOE Building America Solution Center’s excerpt of that table. ENERGY STAR states its retrofit guidance is based on the same IECC table. Your state or city may adopt a different code edition or amend it, so check with the local building department for new work.
Use this table to set your target R-value, then carry that number into the depth formula below.
| Climate zone | Uninsulated attic: bring to (ENERGY STAR retrofit) | Already 3 to 4 in: add (ENERGY STAR retrofit) | New construction ceiling minimum (2021 IECC Table R402.1.3) |
|---|---|---|---|
| Zone 1 | R-30 | R-25 | R-30 |
| Zone 2 | R-49 | R-38 | R-49 |
| Zone 3 | R-49 | R-38 | R-49 |
| Zones 4A and 4B | R-60 | R-49 | R-60 |
| Zones 4C, 5 and 6 | R-60 | R-49 | R-60 |
| Zones 7 and 8 | R-60 | R-49 | R-60 |
Sources: ENERGY STAR, Recommended Home Insulation R-Values (retrofit table for existing wood-framed buildings, accessed October 2026); 2021 IECC Table R402.1.3 ceiling column, as excerpted by the DOE Building America Solution Center (accessed October 2026). Climate zones are assigned by county, so look yours up before you pick a row.
The attic insulation calculator formula (two steps)
The whole calculation is two divisions you can do on a phone. Step one converts your target R-value into a depth in inches. Step two converts your attic area into a bag count. Every manufacturer calculator runs these same two formulas behind the curtain, so knowing them lets you check any number you are quoted.
- Depth needed (inches) = target R-value / R-per-inch of your material. Example: Owens Corning’s AttiCat product data sheet (Pub. No. 10011273-L, May 2026) lists R-49 at 17 inches, which works out to about R-2.9 per inch (49 / 17 = 2.88); run the other way, R-49 / 2.88 = 17 inches. Use your own product’s chart figure, because R-per-inch differs by product.
- Bags needed = attic square footage / coverage per bag at that depth. Coverage per bag is printed on the bag’s chart and drops as target depth rises, so read the row for your R-value, not the top of the chart.
The one trap: coverage per bag is not a fixed number. An AttiCat bag covers 110.7 sq ft at R-19 but only 39.9 sq ft at R-49, per Owens Corning’s data sheet, because you are piling the same material deeper over less floor. Always read the coverage figure from the R-value row you are actually targeting.
How do I measure my attic square footage?
Attic insulation area is the flat ceiling footprint you are covering, measured length times width, not the sloped roof area. For a simple rectangular attic, multiply the two interior dimensions. Subtract nothing for joists (you insulate over them), but do subtract any area you leave clear, such as around recessed can lights not rated IC or a walk board. If you need to cross the attic later, read up on walking in an attic with blown insulation.
For an L-shaped or cut-up attic, break it into rectangles, calculate each, and add them. If you already know your home’s conditioned square footage on a single story, that number is close to the attic footprint and works for a first estimate.
Insulation depth calculator: R-value per inch by material
R-per-inch is the number that converts your target R-value into a depth, and in the table below it runs from about R-2.9 to R-6.5 per inch. Denser and closed-cell products hit the target in fewer inches but cost more per R. Use the R-per-inch column below in step one of the formula.
| Material | R-per-inch (approx.) | Inches for R-49 | Type |
|---|---|---|---|
| Blown fiberglass | About R-2.9 for Owens Corning AttiCat (49 / 17); other products differ | 17 in for AttiCat, per its May 2026 data sheet | Loose-fill |
| Fiberglass batts | About R-3.5 for an Owens Corning EcoTouch R-49 batt (49 / 14) | 14 in for that batt, per the EcoTouch submittal sheet | Batt |
| Blown cellulose | About R-3.2 installed to R-3.6 settled for Greenfiber INS541LD (49 / 15.20 and 49 / 13.68) | 15.20 in installed, 13.68 in settled, per the INS541LD chart | Loose-fill |
| Mineral wool batts | R-3.0 to R-3.3 | 15 to 16 in | Batt |
| Open-cell spray foam | R-3.5 to R-3.9 | 13 to 14 in | Spray |
| Closed-cell spray foam | R-6.0 to R-6.5 | 8 in | Spray |
Two notes that change the depth number. Blown cellulose settles over the first few months, while Owens Corning’s AttiCat fiberglass chart lists settling as negligible, so blow and buy to the installed-thickness spec on the bag chart, which already accounts for any settling. Spray foam is usually installed to a specified thickness by a contractor, so you rarely calculate its bag count yourself.
How many bags of insulation do I need?
Bag count equals your attic square footage divided by the coverage per bag at your target R-value. Coverage per bag is printed on every bag of blown fiberglass or cellulose in a chart that lists square feet covered at each R-value. Bag counts are product-specific. For example, at R-49 Owens Corning’s AttiCat product data sheet (Pub. No. 10011273-L, May 2026) lists 25 bags of its 27.5 lb fiberglass per 1,000 sq ft, and Greenfiber’s INS541LD attic coverage chart (PM-6.3-194 Rev F 03/20) lists 83.5 bags of its 19.05 lb cellulose per 1,000 sq ft, or 80.7 bags in its column adjusted for 2×6 framing on 16-inch centers, all before waste. Check the chart on the bag you are buying.
In those two charts, the cellulose takes more than three times as many bags as the fiberglass for the same attic and R-value, even though it reaches the target in fewer inches. That is not a contradiction: INS541LD goes in at 1.591 lb per sq ft at R-49 against 0.688 lb for AttiCat, and its bag is lighter (19.05 lb against 27.5 lb), so each bag covers 12.0 sq ft instead of 39.9 sq ft. The bag chart, not the inch count, sets your order.
Add 5 to 10 percent for waste, machine loss, and the deeper edges near the eaves. Most rental blowers and stores let you return unopened bags, so round up rather than come up short mid-job.
Worked example: 1,200 sq ft attic to R-49
Take a 1,200 sq ft single-story attic in Climate Zone 3, currently bare to the joists, where ENERGY STAR’s retrofit table recommends R-49. The table below runs the full calculation for three common materials so you can see how depth and bag count move together. Bag figures come from the named manufacturer charts, apply only to those products, and exclude the 5 to 10 percent waste factor.
| Material | Depth for R-49 | Coverage per bag at R-49 | Bags for 1,200 sq ft |
|---|---|---|---|
| Blown fiberglass (Owens Corning AttiCat, 27.5 lb bag) | 17 in | 39.9 sq ft | 30 to 31 bags (25 x 1.2 = 30.0; 1,200 / 39.9 = 30.1) |
| Blown cellulose (Greenfiber INS541LD, 19.05 lb bag) | 15.20 in installed, 13.68 in settled | 12.0 sq ft | 100 to 101 bags (1,200 / 12.0 = 100.0; 83.5 x 1.2 = 100.2) |
| Fiberglass batts (R-49) | 14 in (Owens Corning EcoTouch R-49 batt, per its submittal sheet) | Varies per bundle | By bundle sq ft on label |
Sources: Owens Corning AttiCat product data sheet (Pub. No. 10011273-L, May 2026); Greenfiber INS541LD attic coverage chart, PM-6.3-194 Rev F 03/20. Each chart’s two columns differ slightly because they are rounded, so order the higher count. Greenfiber’s column adjusted for 2×6 framing on 16-inch centers lists 12.4 sq ft per bag and 80.7 bags per 1,000 sq ft, which gives 97 bags for 1,200 sq ft (80.7 x 1.2 = 96.8). Other brands and bag sizes give different counts.
Most home centers loan or rent a blower free when you buy 10 or more bags, which makes blown-in the usual DIY pick for an open attic. Batts make more sense in a small or narrow attic where hauling a blower hose is not worth it, and you size batts by adding up the square feet each bundle covers on its label.
Insulation coverage calculator: reading the bag chart
The coverage chart on a blown insulation bag is the single most important number for your order, and it has three columns you must line up: target R-value, minimum installed thickness, and maximum coverage per bag. Find your R-value row, read across to maximum coverage, and divide your attic area by that number. Ignore the top row’s big coverage figure, which is for a low R-value you are not building to.
- Target R-value: match the row to your climate zone number, for example R-49.
- Minimum installed thickness: the depth to blow at installation, before any settling. Cellulose charts print it beside a lower minimum settled thickness: Greenfiber’s INS541LD chart lists R-49 at 15.20 inches installed and 13.68 inches settled. The 2021 IRC (Section N1101.10.1.1) requires the depth markers stapled to the joists to show this initial installed figure. For how to place and mark them, see our guide to the attic insulation ruler.
- Maximum coverage per bag: the square feet one bag covers at that R-value. This is your divisor.
- Minimum bags per 1,000 sq ft: some charts list this directly, which is the fastest cross-check.
If your attic already has some insulation, subtract its existing R-value from your target before you calculate the top-up. If you know the old layer’s R-value, for example from the label on old batts, and it is dry, undamaged and not compressed, subtract it directly: an R-19 batt under an R-60 target leaves R-41 to add (60 minus 19 = 41). If you can’t identify the old material and it is 3 to 4 inches deep, use ENERGY STAR’s retrofit figures instead: add R-25 in zone 1, R-38 in zones 2 and 3, or R-49 in zones 4 through 8, then read that R-value row on your new product’s chart.
What the manufacturer calculators leave out
Online calculators give you a bag count but usually not the formula behind it, so you cannot sanity-check the result or adapt it when your attic is not a clean rectangle. If a calculator assumes a bare-joist attic and you already have insulation, it will overstate your order for a partial top-up. Running the two-step math yourself catches both problems.
The other omission is air sealing. Insulation slows conductive heat flow, but gaps around the attic hatch, plumbing stacks, and top plates leak air straight through it, and no R-value fixes an air leak. Seal those penetrations first, because a well-sealed R-38 attic outperforms a leaky R-60 one. See our guide on sealing the attic door and hatch for the highest-leak spot most people miss.
How this connects to the rest of your attic project
Insulation depth is one input in a larger attic system that also includes ventilation, material choice, and installed cost. Getting the R-value right does nothing if the attic cannot breathe or if you picked the wrong product for a low-clearance space. The internal guides below cover the pieces this calculator does not.
- For the full R-value target table by material, zone, and thickness, see the insulation R-value chart.
- To pick the right product for your attic, compare options in best insulation for attic and read the deep dive on blown-in insulation cost and R-value.
- Once you know your bag count, the labor and material total is broken down in how much it costs to insulate an attic, and the intake side of the system is covered in attic ventilation.
Ventilation matters here because loose-fill blown into the eaves can block soffit vents, which starves the attic of the airflow that keeps insulation dry and effective. Baffles at each rafter bay keep the vent channel open while you build depth over the ceiling. It is one of the real risks behind the question of whether you can over insulate a house.
Frequently asked questions
How do I calculate how much attic insulation I need?
Run two divisions. Divide your target R-value by the material’s R-per-inch to get the depth in inches, then divide your attic square footage by the coverage per bag at that R-value (from the bag chart) to get the bag count. For a 1,000 sq ft attic to R-49 in Owens Corning AttiCat blown fiberglass, its May 2026 data sheet gives 17 inches deep and 25 bags, before adding a 5 to 10 percent waste factor. Other products list different figures.
How many bags of insulation do I need for a 1,000 square foot attic?
It depends on the product. For example, at R-49 Owens Corning’s AttiCat data sheet (May 2026) lists 25 bags of blown fiberglass per 1,000 sq ft, and Greenfiber’s INS541LD cellulose chart (PM-6.3-194 Rev F 03/20) lists 83.5 bags, or 80.7 adjusted for 2×6 framing on 16-inch centers, before waste. Read the R-49 row on your own bag’s chart. The cellulose takes more bags because its 19.05 lb bag is lighter than AttiCat’s 27.5 lb bag and it goes in denser, so each bag covers less floor.
How many inches of insulation do I need in my attic?
Depth depends on your target R-value and material. For R-49, Owens Corning’s AttiCat blown fiberglass data sheet lists 17 inches, Greenfiber’s INS541LD cellulose chart lists 15.20 inches installed and 13.68 inches settled, and an Owens Corning EcoTouch R-49 batt is 14 inches; other products differ. Divide your target R-value by the material’s R-per-inch to get the exact inch count, then buy to the installed thickness on the bag chart, which already accounts for settling.
What R-value do I need for my attic?
It depends on your climate zone and whether the attic already has insulation. For existing homes, ENERGY STAR’s retrofit table recommends R-30 for an uninsulated attic in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8; with 3 to 4 inches already in place, it recommends adding R-25, R-38, or R-49 for those same zone groups. For new construction, the 2021 IECC (Table R402.1.3) sets ceiling minimums of R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8, though your local code may differ.
How do I read the coverage chart on a bag of insulation?
Line up three columns: target R-value, minimum installed thickness, and maximum coverage per bag. Find your R-value row, read across to coverage per bag, and divide your attic square footage by that number for your bag count. Ignore the chart’s top-row coverage figure, which is for a low R-value you are not building to. Some charts also list bags per 1,000 sq ft directly.
Do I need to add insulation or replace what I have?
In most cases you add over the existing layer rather than remove it, as long as the old insulation is dry and not moldy or rodent-damaged. Subtract the existing R-value from your target before calculating the top-up: for example, an undamaged R-19 batt under an R-60 target leaves R-41 to add. If you don’t know the old layer’s R-value and it is 3 to 4 inches deep, ENERGY STAR’s retrofit table recommends adding R-25 in zone 1, R-38 in zones 2 and 3, or R-49 in zones 4 through 8. Remove and replace only if the old material is contaminated or wet.
Reviewed by The Roofing Brief Team. Last reviewed: October 2026.