Attic Insulation Bags Calculator
Count the bags of blown-in fiberglass or cellulose your attic needs, and the inches to add, from its size, what's already there and your climate zone.
The number on the bag is bags per 1,000 square feet at a settled depth, and it changes with the R-value you're adding, not just the one you're aiming for. This turns your attic, what's already up there and your climate into bags, inches and pounds from the makers' own coverage charts.
Attic floor area
Roughly the footprint of the house under the roof. Subtract the hatch and any chimney or duct chases. A typical single-story home is 1,000–2,000 sq ft; a two-story home's attic is about half its living area.
Existing insulation depth
Push a ruler down to the drywall in a few spots and average. Enter 0 for a bare attic. Older attics often have 3–4 inches; if it's at or below the top of the joists, it's due for more.
Where you live
ENERGY STAR's climate zones set the target. Zone 1 is south Florida and Hawaii; zones 2–3 are the Deep South, Texas, the desert Southwest and southern California; zones 4–8 are everywhere from the Mid-Atlantic, Tennessee and the Pacific Northwest north.
What you're blowing
Both reach the same R-value; cellulose is denser, so it takes more bags and fewer inches, and it weighs more on the ceiling.
Bags of blown fiberglass to buy
28 bags
Takes a bare attic to R-60, in 32-lb bags
The R-value is the bags AND the depth
The chart is a minimum: 28 bags of fiberglass over 1,000 square feet, levelled to 20 inches after settling, is what earns R-60. Fewer bags spread to depth is fluff that settles below the number; the right bags piled unevenly leave cold spots over the bedrooms. Staple the depth rulers to the trusses before you start, blow from the far corners toward the hatch, seal the ceiling and fit eave baffles first, and buy a bag or two extra of a returnable product, because a rental blower and a first-timer never quite hit the maker's coverage.
Target R-value is ENERGY STAR's retrofit table by climate zone; what's there counts about R-3 an inch. Bags and settled depth come from the makers' charts: fiberglass runs 13 bags per 1,000 sq ft at R-30 to 28 at R-60, cellulose 39 to 86. Counts are rounded up to whole bags. 2026 figures.
💡About this calculator▼
The coverage chart on a bag of blown-in insulation answers one question, bags per 1,000 square feet at a given R-value, and it's the R-value you're adding that matters, not the one you're aiming for. An attic with four inches of old insulation in the North needs about R-49 on top, not R-60, and that's five fewer bags per thousand feet.
This counts the bags. Enter the attic floor area, the depth of what's up there now, your ENERGY STAR climate zone and whether you're blowing fiberglass or cellulose, and the calculator works out the R-value to add, reads the manufacturers' own coverage charts for the bags and the settled depth, and tells you what the pile weighs.
Fiberglass and cellulose reach the same R-value with very different piles. Cellulose is denser: about twice the bags, fewer inches, and more than a ton on the ceiling at R-60 over a thousand square feet. Both only deliver the labelled R-value if the bag count and the depth are both met, which is why the result gives you both.
Start with the target. ENERGY STAR's retrofit table sets it by climate zone: an uninsulated attic wants R-30 in zone 1 (south Florida and Hawaii), R-49 in zones 2 and 3 (the Deep South, Texas, the desert Southwest), and R-60 in zones 4 through 8, which is everywhere from the Mid-Atlantic, Tennessee and the Pacific Northwest north.
Credit what's already there. ENERGY STAR puts common attic insulation at about R-3 to R-3.5 per inch; the calculator uses 3, so 4 inches of old fiberglass counts as R-12 and a compressed layer isn't over-credited. The R-value to add is the target minus that. This reproduces ENERGY STAR's own "existing 3–4 inches" column: zones 4–8 come out at add R-49, zones 2–3 at add R-38. If what's there is within R-5 of the target the calculator says you're done, and if it's within R-13, the smallest row on any bag's chart, it says you're close enough: the optional top-up count is shown, but most owners skip a top-up that small and put the money into air sealing.
Read the chart at that R-value. Manufacturers publish bags per 1,000 square feet and settled depth at each R-value; the calculator interpolates between rows. Blown fiberglass in 32-pound bags runs 13 bags at R-30, 16.7 at R-38, 22.2 at R-49 and 28 at R-60, at 10.5 to 20 inches. Blown cellulose in 25-pound bags runs 38.6, 52.6, 70.8 and 86 bags at 8.3 to 16.1 inches settled.
Scale to your attic and round up. Bags per thousand × your square feet ÷ 1,000, rounded up to whole bags; weight is bags × bag weight.
Worked example. A 1,200 sq ft attic with 3.5 inches of old insulation (R-11) in zone 5: add R-49. Fiberglass: 22.2 × 1.2 = 26.6, so 27 bags, about 16.8 inches on top for 20.3 finished, 864 pounds. Cellulose: 70.8 × 1.2 = 85 bags, 13.3 inches settled, 2,125 pounds.
📐How it's calculated▼
Target R = 30 (zone 1), 49 (zones 2–3) or 60 (zones 4–8).
Existing R = existing depth in inches × 3.
R to add = target − existing; 0 if that's under 5.
Bags = (chart bags per 1,000 sq ft at the R to add) × attic sq ft ÷ 1,000, rounded up. Chart rows, fiberglass: R-13 5.4, R-19 8.0, R-22 9.4, R-26 11.2, R-30 13.0, R-38 16.7, R-44 19.7, R-49 22.2, R-60 28.0. Cellulose: R-13 15.2, R-19 22.6, R-22 26.3, R-30 38.6, R-38 52.6, R-44 63.5, R-49 70.8, R-60 86.0. Between rows the calculator interpolates.
Depth to add = the chart's settled thickness at that R: fiberglass 10.5 in at R-30, 13.25 at R-38, 16.75 at R-49, 20 at R-60; cellulose 8.3, 10.4, 13.3, 16.1.
Weight = bags × 32 lb (fiberglass) or 25 lb (cellulose).
Default (1,000 sq ft, bare, zones 4–8, fiberglass): add R-60 → 28 bags, 20 inches, 896 lb.
Top-up (1,200 sq ft, 3.5 in, zones 4–8): R-11 in place, add R-49 → 27 bags of fiberglass at 16.8 in, or 85 bags of cellulose at 13.3 in.
Deep South (1,500 sq ft, 6 in, zones 2–3): R-18 in place, add R-31 → 13.5 bags per 1,000 → 21 bags, 10.8 in.
📎Sources:ENERGY STAR — Recommended Home Insulation R-Values (retrofit table by climate zone),ENERGY STAR — How to Check Your Home's Attic Insulation Level (R per inch, the joist rule),Owens Corning — PROPINK L77 Loosefill Manufacturers Fact Sheet (attic coverage chart, bags per 1,000 sq ft),Greenfiber — SANCTUARY Cellulose Attic Technical Data Sheet (coverage chart, settled thickness)
🔍Finding your inputs▼
Attic floor area: The footprint under the roof, minus the hatch and any chimney or duct chases. A single-story house's attic is about its living area; a two-story house's is about half. Over-measuring is the most common way to buy too many bags.
Existing insulation depth: Push a ruler down to the drywall in three or four places and average. Enter 0 for a bare attic. Older homes usually have 3 to 4 inches of settled fiberglass or cellulose, and ENERGY STAR's rule is that if it's at or below the top of the joists, it's due for more. Matted, damp or rodent-damaged material may be worth less than its depth, or need removing.
Where you live: Three bands of ENERGY STAR's climate map. Zone 1 is south Florida and Hawaii and targets R-30; zones 2 and 3 are the Deep South, Texas, the desert Southwest and southern California and target R-49; zones 4 through 8, from the Mid-Atlantic, Tennessee, Missouri and the Pacific Northwest north, target R-60.
What you're blowing: Fiberglass comes in 32-pound bags and is what the big-box rental blowers are set up for; cellulose comes in 25-pound bags, is denser, takes about twice as many bags and fewer inches for the same R-value, and weighs more on the ceiling. Both use the same blower.
⚠️Special situations▼
The bag says one bag covers 45 square feet at R-49, but the calculator gives me fewer bags than my area divided by 45.
Because you're not adding R-49 from nothing. The bag's coverage is for the R-value you add on top of what's there; if you have four inches of old insulation, you're adding about R-38 to reach R-49 in the South or R-49 to reach R-60 in the North, and the chart row for the smaller R needs fewer bags. Enter the existing depth and the calculator reads the right row.
Cellulose comes out at 85 bags and fiberglass at 27. Is that right?
Yes. Cellulose is about three times as dense as blown fiberglass and comes in smaller bags, so it takes roughly two and a half to three times the bags for the same R-value, in fewer inches: about 13 inches of cellulose against 17 of fiberglass for R-49. The trade-off is weight, over 2,000 pounds versus about 860 for a 1,200 sq ft attic, and a better job of filling odd gaps and slowing air movement. Both use the same rental blower.
I have 12 inches up there already in Texas and it says I'm close enough, but it also shows 32 bags. Which is it?
Both. Twelve inches is about R-36 against a South target of R-49, and the chart says R-13 of cellulose over 2,100 square feet is 32 bags, 800 pounds, 3.6 inches on top, so the count is real. But the last R-13 on a foot of insulation saves the least of any inch you'll ever blow, which is why the verdict is close enough; most owners skip it unless the blower is already rented for another job. Level the low spots with the material you have. The better spend is air sealing: foam and caulk around can lights, the plumbing stack, top plates and the hatch stop heated air going straight into the attic, which no depth of insulation fixes.
My attic is 1,800 square feet but there are two chimneys and a lot of ducts. Do I use the full area?
Use the net area: subtract the hatch, the chimney chases (which need a clearance box anyway), and any platform or duct area you won't blow under. On most attics that's 5 to 10 percent, and it's the difference between the right count and a few bags left over. Also fit eave baffles before you blow so the soffit vents stay open; the strip at the eaves gets less depth and the calculator doesn't deduct for it.
❓Common questions▼
How many bags of blown-in insulation do I need for 1,000 square feet?
It depends on the R-value you're adding. Blown fiberglass in 32-pound bags runs 13 bags per 1,000 square feet at R-30, about 17 at R-38, 22 at R-49 and 28 at R-60; blown cellulose in 25-pound bags runs 39, 53, 71 and 86. A bare attic in the North (R-60) takes 28 bags of fiberglass; one with four inches already there adds R-49 and takes about 22. In the South a bare attic wants R-49, about 22 bags of fiberglass.
How many inches of blown-in insulation do I need?
For the labelled R-value, the depth on the chart: fiberglass about 10.5 inches for R-30, 13 for R-38, 17 for R-49 and 20 for R-60; cellulose about 8, 10.5, 13 and 16 inches settled. ENERGY STAR's rule of thumb is a minimum of R-38, about 13 to 14 inches, in the South and R-49, about 16 to 18 inches, in the North, and its retrofit table goes to R-60 for a bare attic in zones 4 and colder.
What R-value should my attic have?
ENERGY STAR's retrofit table for an uninsulated attic is R-30 in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. If there are already 3 to 4 inches up there, the same table says add R-25, R-38 and R-49 respectively, which is what the calculator reproduces when you enter the existing depth. Existing insulation counts about R-3 per inch, and if you're within one chart row (R-13) of the target the calculator says so rather than sending you for bags.
Is blown-in fiberglass or cellulose better for an attic?
They reach the same R-value; they differ in bags, depth, weight and behavior. Fiberglass is lighter, takes about half the bags, settles very little and is what the free rental blowers are set up for. Cellulose is denser, takes fewer inches, fills gaps and slows air movement better, but weighs about two and a half times as much for the same R and settles, which its chart already accounts for. The calculator gives both so you can compare the pile before you buy.
Why does the bag say to install both a minimum number of bags and a minimum depth?
Because the R-value depends on density as well as thickness. Blow fewer bags fluffed up to the depth and the material settles below the number and insulates less; blow the right bags but pile them unevenly and the thin spots over the bedrooms leak heat. Every manufacturer's chart carries the same warning, and the calculator gives both figures for that reason. Depth rulers stapled to the trusses and blowing from the far corners toward the hatch are how you hit both.
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