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Insulation

Wall Insulation Calculator

Batt counts for stud walls, worked from the framing rather than from square footage alone — with the R-values that actually fit your measured cavity, and the ones that will compress if you force them in.

ft
in

Cavity height is plate to plate, not floor to ceiling. A nominal 8 ft wall frames to about 92⅝ in, which is why batts ship at 93 in and fill that bay in one piece. Measure yours.

in

Measured stud depth, not the nominal label. This tool matches batts on real depth rather than assuming what a “2x4” measures on your job.

Framing
Stud spacing
sq ft

Reduces the area you insulate. It does not reduce the batt count — a bay with a window in it still needs batts above and below, cut from full pieces.

%
sq ft

Read the sq ft per bag off the package you are actually buying. It is packaging, not a physical property — the same R-13 batt ships in bags from roughly 40 to 130 sq ft depending on the retailer and run — so this calculator will not guess it for you.

Buy

15

batts, 15″ wide

4 bags at 40 sq ft

The working

Cavity area: 20 × 92.625″/12 = 154.4 sq ft

− 21 sq ft openings = 133.4 sq ft

+ 10% waste = 146.7 sq ft

Bays: 20 ft × 12 ÷ 16″ o.c. → 15

Batts per bay: 92.625″ ÷ 93″ → 1

15 × 1 = 15 batts, 15″ wide

Bags: 146.7 ÷ 40 sq ft → 4

Fits a 3.5″ cavity: R-11, R-13, R-15

Planning estimate only. Check the result against current product instructions, local requirements, and your own takeoff. See the calculator disclaimer.

Measure the cavity, not the label

Most wall insulation calculators ask what size your studs are and work backwards from a nominal label. That breaks on real jobs. Older framing was cut to different actual dimensions, remodels leave furred-out walls and sistered studs, and a wall someone has already strapped is not the depth its lumber suggests. So this tool asks for the depth you measured, in inches, and matches batts against it.

The same logic applies to height. A nominal 8 ft wall is not 96 inches of cavity — the framed opening between plates is closer to 92⅝ inches, which is precisely why batts are manufactured at 93 inches. Feed a calculator 8 feet and it will tell you to buy two batts per bay when the job needs one. Enter what your tape says between the plates.

R-19 in a 2x6 is the classic trap

R-19 is the most recognisable number in residential insulation and it is routinely put into 2x6 walls, where it does not belong. The batt is 6¼ inches thick. The cavity is 5½. Something has to give, and what gives is the R-value — compressed fiberglass has less trapped air and performs below its label.

R-21 exists for this exact cavity, at 5½ inches. Both figures come from the same Owens Corning product data sheet, which is why this calculator can flag the mismatch instead of quietly letting you order the wrong batt. It shows which R-values clear your measured depth and lists separately the ones that will compress, so the trade-off is yours and visible.

Bays, area, and the two counts

A wall takeoff produces two different numbers that people conflate. The batt count comes from the framing: bays across the run, times pieces needed to fill each bay top to bottom. The bag count comes from area, because that is the unit insulation is sold in. Openings change the second but barely touch the first — a bay with a window in it still needs cripple spaces packed, cut from full batts.

This calculator keeps them separate and shows both in the working. The 10% waste default sits on the area side, where the cutting happens. It is a starting allowance for trimming around openings, blocking, and services, not a code rule — a plain wall with few penetrations may need less, a wall full of pipes and boxes more.

Sources

Grouped by manufacturer or publisher, then by document. Expand a group for the cited document, its verified-on date, and every value taken from it — all of it stays in the HTML either way.

Owens Corning(1 document, 1 value)

Frequently asked questions

How many batts of insulation do I need for a wall?

Count stud bays, not square feet. A 20 ft wall framed at 16 in on center has 15 bays, and a standard framed cavity of about 92⅝ inches takes one 93-inch batt per bay — so 15 batts. Square footage still matters for working out how many bags that is, but the batt count itself comes from the framing, which is why this calculator asks for stud spacing rather than guessing it.

What size insulation do I need for a 2x4 wall?

A 2x4 stud wall has a cavity about 3½ inches deep, and R-11, R-13 and R-15 batts are all made at that thickness. R-15 is the highest R-value that fits a 3½-inch cavity without compression. This calculator asks you to measure the cavity rather than pick a stud label, because dimensions vary with older framing and with what the last remodel left behind.

Can I put R-19 in a 2x6 wall?

You can fit it, but it will not give you R-19. R-19 batts are 6¼ inches thick and a 2x6 cavity is 5½ inches, so the batt compresses — and compressed fiberglass loses R-value. R-21 is the batt actually dimensioned for a 5½-inch cavity. Enter your measured depth above and the tool lists which R-values fit and which will compress.

Why is a wall batt 15 inches wide for 16-inch studs?

Because it sits between the studs, not across them. On 16-inch centers the clear gap is about 14½ inches, so a 15-inch batt friction-fits with a little compression at the edges to hold it. Metal framing is different: those batts are made at the full 16 and 24 inches. Using the wrong set is a real source of short orders, so this calculator switches widths with the framing type.

How many square feet is in a bag of insulation?

There is no single answer, which is why this calculator asks you instead of assuming. Coverage per bag is packaging rather than a property of the material — the same R-13 batt ships in bags ranging from roughly 40 to over 130 sq ft depending on retailer, batt length and run. Read the figure off the bag you are actually buying and enter it; every other number on this page comes from a published manufacturer document.

Should I subtract windows and doors from wall insulation?

Subtract them from the area, which is what drives your bag count, but do not subtract them from the batt count. A bay containing a window still needs insulating above and below it, and those pieces get cut from full batts. That is why the calculator deducts openings from square footage while leaving the bay count alone, and why the waste field defaults to 10%.