Insulation Calculator
Find out how many bags of batt insulation to buy for your attic or walls.
Area to insulate
Results
For a 30 ft × 20 ft attic, at 64 sq ft/bag coverage, you'll need about 10 bags of insulation.
Insulation needed
10 bags
That's a typical amount — in the range of a full attic or the exterior walls of a modest house.
Insulation bags are bulky (though light): this many may not fit in a car in one trip, so consider delivery. Buy it all at once so batt thickness and facing match across the whole surface.
Area to insulate: 600 sq ft
Raw computed amount: 9.38 bags
How this works
The estimate is a two-step division you can check by hand. First, the area to insulate: in attic mode it's simply length × width of the attic floor, because a standard vented attic is insulated at the flat ceiling plane; in walls mode it's the total linear wall length × wall height, minus a standard opening allowance for each door (about 17 sq ft, from a 0.8m × 2.0m opening) and window (about 15.5 sq ft, from 1.2m × 1.2m) you enter — the same structural opening sizes used across BuildyWork's calculators, chosen so you don't have to measure each opening individually at the cost of some precision on unusual openings. Second, that net area is divided by the coverage per bag of your chosen product and rounded up to the next whole bag, since batts are sold by the bag or roll, never by the square foot. Coverage per bag is deliberately an editable input rather than a fixed constant, because it varies more than any other number in this calculation — from roughly 30 sq ft per bag for thick R-38 attic batts to over 100 sq ft for thin R-13 rolls — and the printed figure on your product's bag label is always the authoritative value; our default of 64 sq ft is only a representative mid-range wall batt. Which R-value to buy is a climate decision, not a calculation input: US DOE / Energy Star guidance recommends attic R-30 to R-49 in the warmest zones (1–2), R-38 to R-60 in moderate zones (3–5), and R-49 to R-60 in the coldest zones (6–8) US DOE / Energy Star, Recommended Home Insulation R-Values.
Worked example
An unfinished 30 ft × 20 ft attic being insulated with batts whose bag covers 64 sq ft: the attic floor area is 30 × 20 = 600 sq ft, with nothing to subtract since attic mode has no door or window openings. Dividing by the coverage gives 600 ÷ 64 = 9.375 raw bags — rounded up to 10 bags, since insulation is sold by the whole bag. The leftover from the tenth bag is genuinely useful here: attic jobs always find gaps around the hatch, chases, and end bays.
A second example: exterior walls of a garage
A detached garage where you're insulating 40 linear feet of exterior wall at 8 ft height, with one door and two windows, using the same 64 sq ft/bag product: gross wall area is 40 × 8 = 320 sq ft. Subtracting one standard door (about 17 sq ft) and two windows (about 31 sq ft together) leaves roughly 272 sq ft. That's 272 ÷ 64 = 4.25 raw bags, rounded up to 5. Note how close 4.25 sits to 4 — a slightly generous coverage figure or a skipped window subtraction would have flipped the answer to 4 bags and left the job one batt short, which is exactly why the calculator rounds up rather than to nearest.
Common mistakes
Trusting a generic coverage number instead of the bag label
Coverage per bag ranges from roughly 30 sq ft (thick R-38 attic batts) past 100 sq ft (thin R-13 wall rolls) — a 3× spread. Using this calculator's default without checking your actual product's label can misestimate the order by a factor of two in either direction. The label figure exists precisely for this calculation; always enter it.
Forgetting that higher R-value means more bags for the same area
Upgrading mid-project from R-30 to R-49 attic batts doesn't just change price per bag — each bag covers far less area, so the bag count rises on both fronts at once. Re-run the estimate whenever you change products; the area stays fixed but the division doesn't.
Measuring one wall and multiplying by four
Rooms are rarely square, and whole-house exterior-wall projects never are. Add up the actual linear footage of every wall being insulated rather than assuming symmetry — on an L-shaped plan, the shortcut misses entire wall segments, and the error always lands in the same direction: too little material.
Ignoring existing insulation when topping up an attic
If the attic already has some insulation, you're buying the difference in R-value, not the full target — typically as unfaced batts laid perpendicular over the existing layer. Buying full-target-thickness faced batts both overspends and risks trapping moisture under a second vapor retarder. Assess what's there (depth × R-per-inch of the existing material) before choosing the top-up product.
Not subtracting large openings in walls mode
A patio door or a picture-window wall can be 40+ sq ft of area that needs no insulation. Skipping the door and window counts inflates the wall estimate — usually only by a bag or two, but on a small job that's a meaningful fraction of the order, and the counts take seconds to enter.
Blocking attic ventilation with insulation
Batts pushed tight into the eaves block the soffit vents that keep the attic dry, leading to condensation and, in cold climates, ice dams. Install baffles at the eaves before insulating and keep the vent channel open. This doesn't change the bag count, but it's the most common installation mistake that turns a good estimate into a bad outcome.
Leaving gaps and voids to stretch the material
Insulation's rated R-value assumes complete coverage — even a 5% void area can cut effective wall R-value far more than 5%, because heat takes the uninsulated path. If you come up short, buy another bag rather than spacing batts out; the calculator's whole-bag round-up exists to prevent exactly this temptation.
When this calculator's model stops applying
- Blown-in (loose-fill) insulation: quantity is bags per 1,000 sq ft at a target depth from the bag's coverage chart, not area per bag. Use this tool's area figure, then consult the chart on the loose-fill product.
- Spray-foam projects: estimated in board feet (sq ft × inches of depth) and usually priced by an installer, not bought by the bag. The net-area figure here remains useful as the input to a contractor quote.
- Cathedral ceilings and conditioned-attic rooflines: the insulated plane is the sloped roof deck, larger than the horizontal footprint by the roof's pitch multiplier, and the assembly needs ventilation baffles or unvented-assembly detailing. Compute the sloped area (footprint × pitch multiplier) before dividing by coverage.
- Non-standard framing spacing: batts are made in 15-in and 23-in widths for 16-in and 24-in on-center framing. Old houses with irregular stud spacing force cutting every batt to width, pushing real waste well above what whole-bag rounding absorbs — add a bag or two of margin.
- Rim joists, knee walls, and odd cavities: small areas with high cutting waste and different product choices (rigid foam is common at rim joists because of moisture). Estimate them separately rather than folding them into the main wall length.
- Very large openings beyond standard doors/windows — garage doors, glass walls, pass-throughs: the fixed per-opening subtraction (about 17 sq ft per door, 15.5 sq ft per window) undercounts these. Subtract their measured area from your gross figure manually, or reduce the wall length you enter.
- Soundproofing interior walls: the quantity math is identical, but product selection differs (unfaced or dedicated mineral-wool sound batts, no vapor retarder), and thermal R-value guidance stops being the selection criterion.
- Climate-zone edge cases — the DOE ranges cited on this page are US-specific. Canadian, UK, and EU codes specify different targets in different units (RSI values in metric markets); the bag-count division still works, but take the R-target from your local code.
FAQ
Why does coverage per bag vary so much between products?
Because a bag holds a roughly fixed volume of compressed fiberglass, and thicker batts (higher R-value) mean fewer square feet per bag. An R-13 wall batt bag might cover around 100 sq ft or more, while an R-38 attic batt bag can cover as little as 30–40 sq ft. Batt width matters too — 15-inch batts for 16-inch-on-center framing package differently than 23-inch batts for 24-inch framing. That's why this calculator makes coverage an editable field: the number printed on your specific product's bag is the authoritative figure.
What R-value do I need?
It depends on your climate zone and the surface. US DOE and Energy Star guidance recommends attic insulation of R-30 to R-49 in the warmest zones (1–2), R-38 to R-60 in moderate zones (3–5), and R-49 to R-60 in the coldest zones (6–8); walls are typically R-13 to R-21 depending on stud depth. R-value is a product decision you make before buying — this calculator then tells you how many bags of that product cover your area.
Why does the walls mode subtract doors and windows?
Doors and windows aren't insulated with batts, so their area doesn't consume product. The calculator subtracts a standard-size opening for each one you enter — about 17 sq ft (0.8m × 2.0m) per door and about 15.5 sq ft (1.2m × 1.2m) per window — which is more accurate than estimating from gross wall area alone while staying faster than measuring each opening.
In walls mode, what length do I enter?
The total linear length of all the walls you're insulating, added together. For the full perimeter of a 30 ft × 20 ft room, that's 30 + 20 + 30 + 20 = 100 ft. If you're only insulating the two exterior walls, add just those. The calculator multiplies that total length by the wall height to get gross area, then subtracts the openings you enter.
Should I buy faced or unfaced batts?
Faced batts carry a kraft-paper or foil vapor retarder on one side and are the usual choice for the first layer against conditioned space (facing toward the warm-in-winter side, in most US climates). Unfaced batts are for adding a second layer on top of existing attic insulation — stacking two vapor retarders can trap moisture between them — and for interior walls insulated for sound. The choice doesn't change the quantity math, but it changes which product's coverage number you enter.
Does this calculator handle blown-in (loose-fill) insulation?
No — blown-in fiberglass or cellulose is estimated differently: bags are consumed per square foot at a depth needed to hit a target R-value, using the coverage chart printed on the bag (bags per 1,000 sq ft at each R-value). This tool models batts and rolls, which cover a stated area per package regardless of how you slice them. The area figure it reports is still directly reusable against a blown-in coverage chart.
Do I need to compress batts to fit, and does that change anything?
Avoid it — compressing a batt below its rated thickness reduces its R-value, because the trapped air, not the glass fiber, does most of the insulating. Batts are sized to friction-fit standard cavities (15 in wide for 16-in-on-center studs, 23 in for 24-in centers). If your cavities are shallower than the batt's loft, buy the correct-thickness product rather than squashing a thicker one; the quantity math is the same, but the delivered R-value isn't.
Should I round up or keep the exact bag count?
Round up — this calculator already rounds the raw figure up to the next whole bag, because that's the unit insulation is sold in. Batts get cut around electrical boxes, wires, pipes, and odd-width end cavities, and offcuts are less reusable than they look. Unopened bags are generally returnable, which makes rounding up nearly free insurance against a second store trip mid-job.
Does insulating an attic mean insulating the roof or the floor?
For a standard vented attic, the attic floor (the ceiling of the rooms below) is the insulated plane — that's the area this calculator's attic mode computes from length × width. Insulating at the roofline (a 'conditioned attic') is a different assembly involving rafter-depth batts or spray foam and ventilation baffles; if that's your project, measure the sloped roof deck area instead, which is footprint × pitch multiplier (our roofing tools compute exactly that factor).
What safety gear does fiberglass installation actually require?
Long sleeves, gloves, safety glasses, and at minimum an N95-type dust mask — glass fibers irritate skin, eyes, and airways during cutting and fitting. None of this changes the quantity you buy, but it belongs on the same shopping trip, and attic work adds a second rule: step only on joists or laid boards, never between them, because drywall ceilings do not hold a person.
Can I mix products with different coverage in one estimate?
Run the calculator once per product. A typical whole-house project uses R-13 or R-15 in walls and R-38 or R-49 in the attic — two different coverages, two runs of the tool, two bag counts. Summing areas first and dividing by one blended coverage number gives the wrong answer whenever the products differ.
Can I switch between imperial and metric partway through?
Yes — the unit toggle converts every field you've already entered, including the coverage-per-bag figure, so nothing is lost. Internally the tool always computes in the same canonical units regardless of the display system, so toggling back and forth never introduces rounding drift into your measurements.
Related tools
BTU Calculatorinsulation changes how much cooling a room actually needs — if you're also sizing a room air conditioner, estimate its BTU capacity next.More insulation tools (blown-in loose-fill estimator) are planned but not yet published.