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. A 1,000 sq ft attic targeting R-49 with blown fiberglass at about R-2.6 per inch needs roughly 19 inches and about 22 to 27 bags, depending on brand. 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, and the Department of Energy recommends R-49 to R-60 for the coldest climate zones (6, 7, and 8) and R-30 to R-49 for warmer zones (1 through 5) in vented attics. The exact target is set by your climate zone, not by preference. 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, so you spend less on heating and cooling. The 2021 IECC ties minimum ceiling R-values to climate zone, and many local codes adopt it. Building to code is the floor, not the goal, since attic insulation is one of the cheapest efficiency upgrades per dollar.
Use this table to set your target R-value, then carry that number into the depth formula below.
| Climate zone (examples) | Recommended attic R-value | Rough temperature profile |
|---|---|---|
| Zone 1-2 (South FL, TX coast) | R-30 to R-49 | Hot |
| Zone 3 (GA, AZ, most of CA) | R-30 to R-49 | Warm |
| Zone 4 (VA, TN, OK) | R-38 to R-60 | Mixed |
| Zone 5 (IL, NE, most of NY) | R-49 to R-60 | Cold |
| Zone 6-8 (MN, ND, mountain West, AK) | R-49 to R-60 | Very cold |
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: R-49 target, blown fiberglass at R-2.6 per inch, equals 18.8 inches. Round up to 19.
- 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. A bag that covers 60 sq ft at R-19 might cover only 25 sq ft at R-49, 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.
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 it varies by material from about R-2.2 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 | R-2.2 to R-2.7 | 18 to 22 in | Loose-fill |
| Fiberglass batts | R-2.9 to R-3.8 | 13 to 17 in | Batt |
| Blown cellulose | R-3.2 to R-3.8 | 13 to 15 in | 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 fiberglass and cellulose settle over the first few months, so buy to the installed-thickness spec on the bag chart, which already accounts for 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. As a planning baseline, a 1,000 sq ft attic to R-49 runs roughly 22 to 27 bags of blown fiberglass or 30 to 40 bags of blown cellulose, before waste.
Cellulose needs more bags than fiberglass for the same attic and R-value because each cellulose bag covers less area at depth, even though cellulose reaches the target in fewer inches. That is not a contradiction: fewer inches, but denser material per square foot. 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 5, targeting R-49, currently bare to the joists. The table below runs the full calculation for three common materials so you can see how depth and bag count move together. Bag figures assume typical big-box coverage charts 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 | ~19 in | ~40 to 48 sq ft | 25 to 30 bags |
| Blown cellulose | ~14 in | ~30 to 35 sq ft | 35 to 40 bags |
| Fiberglass batts (R-49) | ~16 in | ~variable per bundle | By bundle sq ft on label |
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, measured after settling, often marked with rulers stapled to the joists.
- 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. A 6-inch layer of old fiberglass is roughly R-13 to R-15, so topping to R-49 means adding only about R-34 to R-36, not the full R-49.
What the manufacturer calculators leave out
Manufacturer and retailer calculators give you a bag count but hide the formula, so you cannot sanity-check the result or adapt it when your attic is not a clean rectangle. They also default to bare-joist attics and often skip the existing-insulation subtraction, which can overstate your order by 30 to 50 percent on 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.
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 blown fiberglass, that is about 19 inches deep and 22 to 27 bags, before adding a 5 to 10 percent waste factor.
How many bags of insulation do I need for a 1,000 square foot attic?
For R-49, a 1,000 sq ft attic needs roughly 22 to 27 bags of blown fiberglass or 30 to 40 bags of blown cellulose, before waste. The exact count comes from the coverage-per-bag figure on the specific product’s chart at the R-49 row. Cellulose takes more bags than fiberglass for the same attic because each bag covers less floor at depth.
How many inches of insulation do I need in my attic?
Depth depends on your target R-value and material. For R-49, expect about 18 to 22 inches of blown fiberglass, 13 to 15 inches of blown cellulose, or 16 to 17 inches of fiberglass batts. 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?
Most attics need R-30 to R-60, set by climate zone. The Department of Energy recommends R-49 to R-60 for cold zones (5 through 8) and R-30 to R-49 for warmer zones (1 through 4) in vented attics. Check your local code minimum, then build to the higher end of your zone’s range since attic insulation is one of the lowest-cost efficiency upgrades per dollar.
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: a 6-inch fiberglass layer is roughly R-13 to R-15, so reaching R-49 adds only about R-34 to R-36. Remove and replace only if the old material is contaminated or wet.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.