Insulation calculator
This divides the area you enter by the coverage you enter — nothing more. It does not set an R-value, judge whether a cavity is deep enough, or account for framing and voids, all of which are governed by your building code. Follow that code and the product's own instructions.
Enter the area you are insulating and the coverage one bag gives at your target thickness. The tool totals the area, adds a small margin for settling and off-ratio spots, divides by the bag coverage and rounds up to whole bags.
Exemple détaillé
An attic 30 ft × 20 ft is 600 square feet. If a bag of loose fill covers 40 sq ft at your chosen depth, that is 600 ÷ 40 = 15 bags before waste. Add 5% for settling and rounding and you are buying 16 bags. Aim for a deeper R-value and the coverage per bag drops, so the count climbs.
In metric: an attic 9 m × 6 m is 54 m². At 3.7 m² a bag that is 54 ÷ 3.7 = 14.6 bags; with 5% it is 15.3, so 16 bags.
Sur le chantier
Reading the chart on the bag. Blown-in bags do not say "covers 40 ft²"; they carry a chart of bags per 1,000 ft² at each depth. Find the row for the depth your job calls for — say it reads 28 bags per 1,000 ft² — and the coverage per bag is 1,000 ÷ 28 = 35.7 ft². A 1,400 ft² attic is then 1,400 ÷ 35.7 = 39.2, × 1.05 = 41.2 → 42 bags. The chart is the manufacturer's, the depth is the plan's; this page only divides. A metric bag's chart reads per 100 m²: 30 bags per 100 m² is 100 ÷ 30 = 3.33 m² a bag.
Batts by the bundle. A 40 ft wall, 8 ft high, framed at 16 in centres is 320 ft² of cavity, near enough. If the bundle label says 77.5 ft², then 320 × 1.05 ÷ 77.5 = 4.3 → 5 bundles. Type the bundle coverage in the per-bag field and it counts bundles; the batt width matches the framing (15 in for 16 in centres), so the area method holds. In metric: 12 m × 2.4 m is 28.8 m²; a bundle labelled 7.2 m² gives 28.8 × 1.05 ÷ 7.2 = 4.2 → 5 bundles.
Topping up an attic. Push a ruler into the existing fill in five or six places and average it. The bag chart is for the depth you are adding, not the finished depth — read the row for the difference. Six inches there and twelve wanted is a 6 in row, not a 12 — in metric, 150 mm there and 300 mm wanted is the 150 mm row.
What went wrong?
Quick referencefor insulation
- Couverture par sac
- 1,000 ÷ (bags per 1,000 ft²) from the row for your depth
- Coverage per bag, metric
- 100 ÷ (bags per 100 m²) from the row for your depth
- Twice the depth
- half the coverage, twice the bags
- Batt widths
- 15 in for 16 in centres · 23 in for 24 in centres (product sizes)
- Wall cavity area
- wall length × height, openings off
- Attic
- the attic floor, not the roof — the footprint of the house, overhangs out
- Métrique
- 1 m² = 10.76 ft² · a bag at 3.7 m² ≈ 40 ft²
Waste & overagewhat to add, and why
- Blown-in installers add 5% for the settling the machine does not account for and for the bits that go down the wall cavities; batts carry 5–10% because every cut around a pipe wastes a strip. The waste field starts at 5%.
- Bag coverage drops as depth goes up, so if the customer changes the R-value target, re-read the chart line for the new depth before you order.
Mistakes to catchbefore you order
- Using the coverage from the bag for a different depth than the one you are installing — the chart on the bag is a table by depth, and coverage at 6 in is roughly half the coverage at 3 in.
- Measuring the attic floor and forgetting the sloped bits at the eaves that also need fill.
- Counting batts by the square-foot number on the package when they come in a specific width — a 16 in batt does not fit a 24 in bay without cutting, and the cut-offs are waste.
What an inspector looks forthe building inspector, at the insulation and vapour-barrier call
On this part of the job
- The insulation value achieved in each assembly against what the energy code or the energy report requires — the number is in your own code book and on the permit drawings.
- Fit: batts cut around wiring and plumbing rather than compressed over them, no gaps at the edges, and the full depth maintained in the corners of a rafter bay.
- Air barrier and vapour control layer continuous, sealed at every penetration, at the top and bottom plates and at every service box.
- Attic work: the marked depth achieved right out to the edges, baffles at the eaves keeping the vent path open, and insulation kept off anything that gets hot.
- Blown insulation depth verified against the coverage chart on the bag — bag count for the area is exactly what that chart is for, and the inspector can do the same arithmetic you did.
Have this ready
- The bag count you used and the manufacturer's coverage chart.
- Attic depth markers installed and legible.
- The energy compliance paperwork the permit was issued against.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La formule
bags = (area × (1 + waste)) ÷ coverage per bag, rounded up
The coverage already carries the thickness inside it — a bag that covers 40 sq ft deep enough for a high R-value covers far more when spread thin. That is why the tool asks for coverage rather than depth: it keeps the one variable that changes with your code requirement in your hands.
Questions
- How many bags of insulation do I need?
- Divide the area you are insulating by the coverage one bag gives at your chosen thickness, then round up. Coverage is printed on the bag and falls as the target depth rises, so it is the number that drives the answer and it is editable here. Enter the area and the bag coverage and the tool counts the bags.
- Does this tell me what R-value to use?
- No — and that is deliberate. The required R-value for a wall, ceiling or floor is set by your building code and climate zone, not by a calculator, so this tool never suggests one. You decide the R-value from your code, read the coverage the bag gives at that value, and enter it here. The arithmetic is all this does.
- How is blown-in coverage different from batts?
- Blown-in loose fill is sold by the bag, and the bag states how many square feet it covers at each settled thickness — that is the coverage figure to enter. Batts are sold by the bundle with a stated square footage instead; enter that bundle coverage in the same field and the count still works.
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