Enter your attic or room dimensions — or the square footage straight up — and
get the total area plus how many bags of blown-in insulation or rolls of batts it takes.
Coverage per bag changes with the R-value you're targeting, so the tool pre-fills a
sensible default by type and lets you override it with the exact figure printed on your
product's bag.
Read this first
Coverage per bag is product-specific and drops as the target R-value rises — a single
bag covers far less area at R-49 than at R-19. This tool computes your area
precisely, then divides by a coverage figure. The defaults below are typical starting
points; the only number you should trust for an actual purchase is the
coverage printed on the bag or roll you're buying, for your target
R-value. Always buy a little extra and check that figure before you go to the register.
The calculator
Estimate area and bags or rolls
Pick your insulation type (it sets a default coverage you can edit), choose how you want to enter area, add as many spaces as you need, and you'll get the total square footage plus the number of bags or rolls required.
Default for the selected type. Override this with the figure printed on your product's bag for your target R-value — it changes with R-value.
A small buffer (5–10%) covers trimming, gaps, and odd corners.
Total area
Coverage used
Units before overage
Overage applied
The math, honestly
How the bag count is figured
It's two simple steps. First, area: for a rectangular space the area
in square feet is length × width, with both in feet, and multiple
spaces just add together. If you already know the square footage, enter it directly.
Second, bags or rolls: divide the total area by the coverage per
bag or roll and round up, because you can't buy a fraction of one —
units = ⌈ area ÷ coverage ⌉. A 40 ft × 30 ft
attic is 1200 sq ft; with blown-in fiberglass at 40 sq ft per bag that's
⌈1200 ÷ 40⌉ = 30 bags.
Why coverage isn't fixed: the coverage per bag drops as your target
R-value rises, because the same material is spread over a smaller area to reach the
greater depth. That's why this tool only pre-fills a typical default and asks you to
replace it with the number on your bag. A small overage (5–10%) covers
trimming, gaps, and the odd corner that eats more than expected.
Typical coverage by type and R-value
Approximate coverage per bag or roll for common insulation types and target R-values.
These are typical figures only — always use the coverage printed on YOUR
product's bag, since it changes with the target R-value and varies by brand and
product line. Notice how the same bag covers less area as R-value climbs.
Type
Sold as
~R-19sq ft / unit
~R-30sq ft / unit
~R-49sq ft / unit
Blown-in fiberglass
per bag
~60
~40
~25
Blown-in cellulose
per bag
~55
~35
~22
Batt rolls
per roll
~40
~40
~30
Typical, rounded figures for general guidance — not a spec. Blown-in coverage falls
steeply with R-value because more material is packed in per square foot. Batts are sized
by thickness, so a single R-value spans a roll; higher-R batts are thicker and cover less.
The calculator's defaults (40 / 35 / 40 sq ft) sit near the R-30 column. Confirm against
the bag or roll you buy.
Attic R-value, by climate (general guidance)
Rough US Department of Energy attic targets by climate, as general information,
not a code requirement. The right number for your home depends on your climate
zone, your local building code, and whether you're insulating new construction or topping
up. Check your local code and a DOE or Energy Star zone map before buying.
Climate
Rough regionUS, general
Attic R-valuetypical target range
Hot / mild
Deep South, Gulf Coast
R-30 to R-49
Mixed
Mid-Atlantic, Midwest
R-38 to R-60
Cold
Northern states, mountains
R-49 to R-60
Ranges are approximate and span overlapping DOE recommendations; R-38 is a common
middle-of-the-road attic target. Always defer to your local building code and an official
climate-zone map — these rows are background orientation only.
Reading the result well
A bag count is only useful if you act on it sensibly. Four things worth knowing before
you buy.
The coverage number is everything
Your area is fixed by your rooms, but the bag count swings entirely on coverage per bag — and that coverage falls as R-value rises. A bag that covers 60 sq ft at R-19 might cover only 25 sq ft at R-49. Set your target R-value first, then read the matching coverage off the bag and type it into the tool. Don't trust a single generic number across R-values.
Blown-in settles — the bag already accounts for it
Loose-fill insulation compacts after install, so manufacturers print both an installed and a settled thickness, and calibrate the bag count to hit the rated R-value after settling. If you install to the labeled depth and use the labeled number of bags, you land at the right R-value. This is one more reason to follow the bag over a rule of thumb.
Match the type to the job
Blown-in fiberglass or cellulose is fast for open attics and conforms around obstructions; it needs a blower (often a free loaner with bulk purchase). Batt rolls are simpler for accessible walls and floors with regular framing and need no machine. The right choice changes how coverage is sold — per bag for blown-in, per roll for batts.
Buy whole bags — and add a small buffer
You can't buy a fraction of a bag, so the calculator rounds every count up. A 5–10% overage covers trimming batts, filling gaps, and the corners that eat more than the math suggests. A leftover partial bag is cheap; coming up short and leaving a thin spot that bleeds heat is the costlier mistake.
Where to buy
Got your numbers? Here's where to pick up what you need:
The terms behind the calculator, in plain English. These are background definitions, not
engineering specifications — follow your local building code and the manufacturer's
coverage chart for the real thing.
R-value
A measure of a material's resistance to heat flow — higher means better insulation. You raise R-value by installing thicker, denser material, which is why coverage per bag falls as R-value climbs. Target R-value is set by your climate zone and local code.
Coverage per bag (or roll)
How much area, in square feet, one bag of blown-in or one roll of batts covers at a stated R-value. It is the single most important number for counting units, and it is product- and R-value-specific. The bag prints a coverage chart; always use that figure rather than a generic estimate.
Blown-in (loose-fill)
Loose fiberglass or cellulose blown into an attic or cavity with a machine. It conforms around obstructions and fills irregular spaces, and is sold per bag. It settles over time, which manufacturers account for on the bag.
Cellulose
A blown-in insulation made largely from recycled paper treated for fire and pests. It's denser than blown fiberglass, often covers slightly less area per bag at the same R-value, and tends to settle more — again, the bag's coverage already factors settling in.
Batt
A pre-cut blanket of fiberglass or mineral wool sized to fit between standard joists and studs, installed by hand and sold per roll or per pack. Batts come faced (with a vapor barrier) or unfaced; use unfaced material when adding a second layer on top of existing insulation.
Settling
The gradual compaction of loose-fill insulation under its own weight after installation. Manufacturers list both installed and settled thickness, and calibrate the printed bag count to reach the rated R-value once settled — so following the bag gets you there.
Climate zone
A region grouping used by the US Department of Energy and building codes to recommend insulation levels. Colder zones call for higher attic R-values (often R-49 to R-60); milder zones less (around R-30 to R-49). Check the official zone map and your local code for your target.
Overage (waste factor)
A small percentage added on top of the calculated units — typically 5 to 10% — to cover trimming, gaps, and corners that consume more than the raw math. Cheap insurance against the costlier problem of a thin, heat-leaking spot.
Frequently asked
Divide the square footage you're covering by the coverage per bag printed on your product, then round up. A 40 ft × 30 ft attic is 1200 sq ft, and blown-in fiberglass at 40 sq ft per bag means 1200 ÷ 40 = 30 bags. The catch is that coverage per bag isn't fixed — it falls as the target R-value rises, because more material packs into each square foot. Always use the coverage on the bag you actually buy for your target R-value rather than a generic number. Try it in the calculator.
Blown-in is loose fill — fiberglass or cellulose — blown into attics and cavities with a machine, so it conforms around obstructions and fills irregular spaces well. Batts are pre-cut blankets sized to fit between standard joists and studs, installed by hand. Blown-in is usually faster for open attics and easier in awkward spots; batts are simpler for accessible walls and floors with regular framing and need no machine rental. Coverage is sold per bag for blown-in and per roll for batts.
As general guidance, US Department of Energy recommendations land around R-30 to R-49 for attics in milder southern climates and roughly R-49 to R-60 in colder northern climates; R-38 is a common middle-of-the-road target for many existing homes. This is general information, not a code requirement — the right number depends on your climate zone, your local building code, and whether you're insulating new construction or topping up. Check your local code and a DOE or Energy Star zone map for your area first.
Coverage per bag drops as the target R-value rises. R-value measures resistance to heat flow, and you raise it by installing the material thicker and denser. A bag that covers about 60 sq ft at R-19 might cover only about 40 sq ft at R-30 and even less at R-49, because the same bag is spread over a smaller area to reach the greater depth. That's exactly why this calculator asks you to enter the coverage from your bag at your target R-value rather than assuming one fixed number.
Loose-fill insulation settles after installation as it compacts under its own weight, especially cellulose. Manufacturers account for this by listing two thicknesses on the bag — the initial installed thickness and the settled thickness — and the coverage and bag count printed on the package are calibrated to deliver the rated R-value after settling. So if you install to the labeled installed depth and use the labeled number of bags, you end up at the right R-value once it settles. One more reason to trust the coverage on the bag over a generic estimate.
For a simple rectangular attic, multiply length by width in feet to get the floor area — a 40 ft × 30 ft attic is 1200 sq ft. For an L-shaped or irregular attic, split it into rectangles, calculate each, and add them together. This calculator lets you add multiple spaces, so you can sum several rooms or attic sections in one pass. Measure the area you're actually covering with insulation — the floor plane — not the sloped roof surface.
In an attic you can usually add new loose-fill or unfaced batts on top of existing insulation to raise the R-value, as long as the old material is dry and not compressed or moldy. The new R-value is roughly the old plus the new added on top. Don't add a second layer of faced batts over the first, because the vapor barrier in the upper layer can trap moisture — use unfaced material for the top layer. When topping up, base your bag count on the additional coverage needed to reach your target, not the full target from scratch.