Insulation Calculator

Calculate how many packages and batts of fiberglass insulation you need for a wall, floor or attic from the area, framing spacing, R-value and coverage.

By CalcBuilt Editorial TeamUpdated September 11, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To find how much insulation you need, multiply wall length by height, subtract doors and windows, add 10% for waste, then divide by the coverage printed on the package. A 40 x 8 ft wall with 40 sq ft of openings has 280 sq ft to insulate; with waste that is 308 sq ft, or 8 packages of R-13 at 40 sq ft each (32 batts of 15 x 93 in).

Your measurements

One wall: area is wall length times height, minus each door and window.

Wall or attic lengthftWall height or attic widthft8 ft walls take 93 in batts; a 9 ft wall needs 105 in batts or a roll cut to length.Doors and windowssq ftTotal area of openings to subtract. A standard door is about 20 sq ft, a window 10 to 15 sq ft. For an attic enter 0; the 40 sq ft default is for a wall.Framing spacingInsulationCoverage per packagesq ftPrinted on the bag, not the batt count. It runs from about 40 sq ft for thick R-38 batts to about 100 sq ft for R-13.Waste allowance%Waste is cuts: 10% for a wall with outlets, pipes and odd bays, 5% for an open attic, 15% for a knee wall or a floor with blocking.

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Packages to buy8 packagesAll results

Results

Packages to buy: 8 packages; At 40 sq ft per package
Area to insulate: 280 sq ft; 308 sq ft with 10% waste
Batts needed: 32 batts; 15 in x 93 in batts, 9.7 sq ft each
R-value: R-13; 3.5 in thick, for existing 2 x 4 exterior walls

Insulation formula

Area to insulate = Length (ft) x Height (ft) - Openings (sq ft)
Order area = Area to insulate x (1 + waste %)
Packages = Order area / coverage per package
Batts = Order area / (batt width x 93 in / 144)  (15 in batt = 9.69 sq ft, 23 in batt = 14.85 sq ft)

Worked example

An attic 30 x 40 ft with joists 24 in on center, insulated with R-38 batts that cover 48 sq ft per package: 30 x 40 = 1,200 sq ft. With 10% waste, 1,320 sq ft. 1,320 / 48 = 27.5, so buy 28 packages.

In batts: a 23 x 93 in batt covers 14.85 sq ft, and 1,320 / 14.85 = 88.9, so about 89 batts. That agrees with 28 packages if each holds 3 or 4 batts.

Quick reference

Packages and batts needed by area (10% waste, 40 sq ft per package)
Area to insulate	With waste	Packages	Batts, 16 in OC	Batts, 24 in OC
100 sq ft	110 sq ft	3	12	8
200 sq ft	220 sq ft	6	23	15
400 sq ft	440 sq ft	11	46	30
600 sq ft	660 sq ft	17	69	45
800 sq ft	880 sq ft	22	91	60
1,000 sq ft	1,100 sq ft	28	114	75

How to calculate insulation

Measure the length and height of each wall, or the length and width of the attic floor, and multiply for square feet. Subtract doors and windows for walls; for an attic subtract nothing but the chimney and stairwell. Add 10 percent for waste, then divide by the coverage figure printed on the package. For an open attic over about 800 sq ft, blown-in is cheaper per R, covers joists and gaps better, and the blower rental is free with 10 or more bags.

A batt for 16 inch framing is 15 inches wide, one for 24 inch framing is 23 inches wide, and both come 93 inches long for an 8 foot wall. A 9 foot wall needs 105 inch batts.

Choosing an R-value

Existing 2 x 4 walls take R-13 or R-15; new construction in zone 3 and up needs R-20 or R-13 plus R-5 continuous foam (2021 IECC Table R402.1.3). A 2 x 6 wall takes R-21; R-19 is a floor batt that compresses to about R-18 in 5.5 inches, and floor batts need wire supports every 12 to 16 inches. Attics need R-49 in zones 2 and 3 and R-60 in zones 4 through 8, or R-38 and R-49 under 2015 and older codes; a second layer goes unfaced and perpendicular to the joists.

Air seal before you insulate: foam the top plates, wire and pipe penetrations and the chimney chase, and staple soffit baffles in every rafter bay. Kraft facing goes toward the warm-in-winter side and must be covered by drywall (IRC R302.10), so garages, basements and crawlspaces get unfaced or FSK-faced batts; never stack two vapor retarders. Keep batts 3 inches off non-IC recessed cans, split them around wires, and insulate and weatherstrip the attic hatch.

What this calculator leaves out

The result is batts only. Not included: canned foam and caulk for air sealing, soffit baffles (one per rafter bay), wire supports for floor batts, and staples for kraft flanges. Also drywall or a vapor retarder over unfaced batts where required, a hatch cover, and attic blown-in (see the blown-in calculator).

Mistakes that cost money

Kraft-faced batts left exposed in a garage or basement are a fire hazard (IRC R302.10); use unfaced or FSK-faced, or cover with drywall.
Insulation stuffed into the eaves with no baffles blocks the soffit vents; the attic sweats, the sheathing molds and shingles cook.
Batts over a non-IC recessed can trip its thermal cutout or start a fire; keep 3 in clear or swap in an IC-rated can.
R-19 compressed into a 2 x 4 wall performs at about R-13 and costs more; buy R-13 or high-density R-15.
Batts in a 1,200 sq ft attic cost more per R than blown-in and leave gaps at every joist; blown-in with the free blower wins.
Insulating without air sealing first: fiberglass over an open top plate gap is a filter, not a barrier; foam plates and penetrations first.

Key facts

The 2021 IECC (Table R402.1.3) requires R-49 attics in climate zones 2 and 3 and R-60 in zones 4 through 8; states on 2015 or older codes use R-38 and R-49.
A standard 3.5 in batt is R-13 (R-15 high density); R-21 is the 5.5 in batt made for 2 x 6 walls, and a 6.25 in R-19 compressed into 2 x 6 performs at about R-18.
Batts are 15 in wide for 16 in framing and 23 in wide for 24 in framing. They come 93 in long for an 8 ft wall or 105 in for a 9 ft wall.
Compressing a batt into a shallower cavity lowers its R-value; a 6.25 in R-19 squeezed into a 3.5 in wall performs at about R-13.
Fiberglass batts have an R-value of roughly 3.1 to 3.4 per inch; blown cellulose is about 3.2 to 3.8 per inch.

Frequently asked questions

How much insulation do I need for a 12 x 12 room?

Measure the exterior walls only. One 12 x 8 ft wall is 96 sq ft; with a window subtracted and 10% waste, about 95 sq ft, or three packages of R-13 at 40 sq ft each.

What R-value do I need in my attic?

2021 IECC calls for R-49 in climate zones 2 and 3 and R-60 in zones 4 through 8; states still on 2015 or older codes use R-38 and R-49. Adding to an existing attic, lay unfaced batts or blow cellulose over the old layer until a ruler shows the target depth.

Should I use faced or unfaced insulation?

Faced (kraft) in exterior walls and floors where drywall or subfloor will cover it, facing toward the living space. Unfaced or FSK-faced anywhere it stays exposed, such as a garage, basement or crawlspace, because kraft paper burns (IRC R302.10). A second attic layer is also unfaced so there is only one vapor retarder.

Can I use R-19 in a 2 x 4 wall?

You can, but compressing a 6.25 in batt into a 3.5 in cavity drops its performance to about R-13, so it costs more and gains nothing. Use R-13 or high-density R-15 instead.

Sources and references

Recommended Home Insulation R-Values (ENERGY STAR)
Consumer Guide to Home Insulation (Energy Saver) (U.S. Department of Energy)
Chapter 11 Energy Efficiency, 2021 IRC (ICC)
PINK Next Gen Fiberglas Insulation product page (Owens Corning)

Next steps for this project

The usual order for a insulation & hvac project. See all insulation & hvac calculators.

Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.