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 9, 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).

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 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. Coverage varies a lot between products, from about 40 sq ft for a bag of R-13 to 90 sq ft or more for thin, wide batts.

The calculator also converts the area to batts so you can compare products sold per piece. A batt for 16 inch framing is 15 inches wide, one for 24 inch framing is 23 inches wide, and both are 93 inches long to fit between the plates of an 8 foot wall.

Choosing an R-value

Match the batt to the cavity. A 2 x 4 wall takes R-13 or R-15, a 2 x 6 wall takes R-19 or R-21, and floors over a crawlspace usually take R-19 to R-30. Attics have no cavity limit, so use R-38 or higher; in cold climates R-49 to R-60 is recommended, which usually means layering batts or adding blown-in insulation over the joists.

Air seal before you insulate. Fiberglass slows heat flow but does not stop air, so caulk gaps around plates, wiring and plumbing penetrations first. Fit batts snugly without compressing them, and split them around wires and pipes rather than stuffing them behind.

Key facts

The US Department of Energy recommends R-38 to R-60 in attics and R-13 to R-21 in walls for most climate zones.
A standard 3.5 in fiberglass batt is R-13; a 2 x 6 wall takes an R-19 or R-21 batt at 5.5 to 6.25 in.
Batts are 15 in wide for 16 in framing and 23 in wide for 24 in framing, and 93 in long for an 8 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?

R-38 is the minimum in most of the US, and R-49 to R-60 is recommended in cold northern climates. Check the DOE zone map for your area.

How many batts are in a package?

It depends on thickness. A bag of R-13 often holds 10 to 12 batts covering about 40 sq ft on 16 in framing; a bag of R-38 might hold only 4 or 5 batts. The coverage in square feet is printed on every package.

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.