Wall insulation calculator

Measure the wall run, its height and the openings you subtract: you get the net square footage, the batt bundles to order and the R-value the stud cavity can hold.

Confirm coverage-per-bag, R-per-inch and set yield against the exact product you buy and order a little extra (~5–10%) for framing, gaps and settling. Coverage, R/inch and set yields vary by product and brand — read the bag/kit and the data sheet.

Calculator

ft
Total run of wall you are insulating (add the runs for a room).
ft
Floor to top plate — usually 8, 9 or 10 ft.
ft²
Total area of windows and doors to subtract. A 3×5 window is 15 ft².
ft²/bundle
Off the bag: R-13 3.5" ~88 ft², R-15 ~67, R-21 ~58 (labeled).
in
2×4 = 3.5", 2×6 = 5.5" of actual cavity depth.
/in
Fiberglass batt ~3.1–3.4, mineral wool ~3.0–3.3 (labeled).
Result
Batt bundles needed4 bundles
Net wall area280 ft²
Cavity R-value (3.50")R-11.2

40 ft × 8 ft minus 40 ft² of openings is 280 ft² net — about 4 bundles. A 3.50" cavity holds about R-11.2 (or an R-13/R-15 batt). Confirm batt coverage on your bag.

Formula

net_area = wall_length_ft × height_ft − openings_ft²

bundles = ceil( net_area ÷ coverage_per_bundle )

cavity_R = cavity_depth_in × material_R_per_inch

Subtract the windows and doors before you buy — paying for batts you will cut out is the most common over-order on a wall. The bundle count rounds up because you buy whole bundles, and the cavity R tells you the most a filled stud bay can deliver at that depth (before you subtract for the framing itself).

Worked example

A 40 ft run of 8 ft wall with 40 ft² of windows and doors, R-13 batts covering 88 ft²/bundle, in a 2×4 (3.5") cavity at R-3.2/in:

Net area: 40 × 8 − 40 = 280 ft². Bundles: ceil(280 ÷ 88) = ceil(3.18) = 4 bundles. Cavity R: 3.5 × 3.2 = R-11.2 (an R-13 or R-15 batt fills the same bay).

So order 4 bundles for the room. The R-11.2 is the fill at that R/inch; a purpose-made R-13 or R-15 batt is sized to the 3.5" cavity and gets you a little more without over-stuffing.

What to subtract & how to fill it

Subtract the openings, but not too much. Take out full window and door areas, yes — but you still insulate the narrow strips above and below windows and the cavities beside doors. If you round openings up aggressively you will come up a bundle short. When in doubt, keep the extra bundle; batts store dry and flat.

Match the batt to the cavity, do not compress. An R-19 batt jammed into a 3.5" 2×4 bay is worse than an R-13 sized for it — compressing insulation loses R fast. Use R-13 or R-15 in a 2×4 wall and R-19, R-21 or R-23 in a 2×6. The tool’s cavity-R line assumes the fill matches the depth.

Cut clean and fill the gaps. Split batts around wiring and boxes rather than mashing them behind, and cut a hair oversized so friction-fit batts do not slump. The voids at the top and bottom plates and beside the king studs are where walls leak R — a can of spray foam at those seams buys more than an extra bundle.

Blown or dense-pack is an option too. This tool sizes batts, but a retrofit wall you cannot open is often dense-packed with cellulose or blown fiberglass through small holes. For that path, price it as bags with the blown-in calculator instead of bundles.

Reference table

Labeled batt coverage per bundle by R-value — read the figure off the bundle you actually buy.

Batt R-valueCoverage per bundle
R-1388 ft²/bundle
R-1567 ft²/bundle
R-1975 ft²/bundle
R-2158 ft²/bundle
R-3058 ft²/bundle
R-3840 ft²/bundle

Higher R = thicker batt = fewer ft² per bundle. Labeled planning typicals.

Frequently asked questions

How many bundles of insulation do I need for a wall?
Take the wall length × height, subtract the window and door openings, then divide by the coverage per bundle and round up. A 40 ft × 8 ft wall (280 ft² after 40 ft² of openings) with R-13 batts at 88 ft²/bundle needs ceil(280 ÷ 88) = 4 bundles. Read the coverage off your own bag.
What R-value fits in a 2×4 vs a 2×6 wall?
A 2×4 has a 3.5-inch cavity — R-13 or R-15 batts. A 2×6 has a 5.5-inch cavity — R-19, R-21 or R-23. Do not force a thicker batt into a shallower bay; compressing it loses R-value. The cavity-R line here assumes the batt matches the depth.
Do I subtract the windows and doors?
Yes — subtract the full opening area so you do not buy batts you will cut out. But remember you still insulate the small cavities above and below windows and beside doors, so keep a spare bundle rather than shaving it too close.
Why is the cavity R lower than the batt R-value?
The tool multiplies your material R/inch by the actual cavity depth, so a 3.5-inch fill at 3.2/in reads R-11.2. A labeled R-13 or R-15 batt is engineered to hit its rating in a 3.5-inch bay — use the batt R for the product and the cavity-R line as a physics sanity check.
Can I dense-pack instead of using batts?
Yes. Closed walls you cannot open are usually dense-packed with cellulose or blown fiberglass through drilled holes. For that, size it in bags with the blown-in insulation calculator; this tool prices batt bundles for open framing.

Related calculators

Batt insulation calculatorWork out how many batt insulation bundles to buy. Enter the area and the coverage per bundle, add a waste allowance, and get the bundle count rounded up.Blown-in insulation calculatorCalculate how many bags of blown-in insulation you need. Choose the coverage-per-bag-at-R chart or the depth (board-feet) method for your attic square footage.R-value calculatorAdd insulation layers in series to get the total assembly R-value and U-value. Enter thickness and R per inch for each layer, add air films, done.Insulation cost per square footWork out insulation cost per square foot from your area and your quoted $/ft2. Free calculator with the formula, a worked example and labeled per-foot bands.