2440
Attic insulation calculator
Put your attic length, width, depth and waste in: a 30 ft by 20 ft attic at 12 in deep needs 25 bags exact, 28 bags with 10% waste, at 48 sq ft a bag at 6 in depth.
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Attic Insulation Calculator: 28 Bags for a 30x20 ft Attic
On the defaults — a 30 ft by 20 ft attic, 12 in deep, 10% waste — you need 28 bags of insulation. This is the attic insulation tool: measure the attic once, allow for the joists, and buy the bags once — a half-insulated attic waits for no second delivery.
What does attic insulation needed 28 bags mean?
It means twenty-eight bags insulate a 30 ft by 20 ft attic at 12 in deep, with room for joists and gaps. The attic itself is 600 square feet, since 30 times 20 is 600; one bag covers 48 square feet at 6 in deep, so it covers only 24 square feet at 12 in deep, and 600 divided by 24 is 25 bags exact, with 10% waste for joists and gaps that becomes 27.50, rounded up to 28 whole bags because bags are sold by the bag, not by the fraction. The spare bags are not spare stock exactly — they are the bags the joists, the gaps and the settling consume, so an attic insulated to the exact bag runs short before the last bay.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Attic area | length × width | 600 sq ft — 30 × 20. |
| Bag coverage | 48 sq ft at 6 in ÷ (depth ÷ 6) | 24 sq ft — at 12 in deep. |
| Bags | area ÷ coverage, plus waste, rounded up | 28 bags — 25 × 1.10, rounded up. |
Three things to check before you trust the number
First, check the coverage on the bag label you are actually buying: 48 sq ft at 6 in is the common figure, but a denser bag covers less per bag and raises this attic past 28 bags, while the attic area never changes. Second, remember the count follows the area times the depth, not the area alone: a 30 ft by 20 ft attic is 600 sq ft, but at 12 in deep it needs twice the bags it needs at 6 in. Third, judge the waste honestly: a plain attic fits 10%, but an attic cut with joists, wires and vents can want 15%, since every gap takes a bag. The R-value, the vapor barrier and the ventilation are planned separately — see the insulation cost calculator for the whole job priced out.
Worked example
30 ft long, 20 ft wide, 12 in deep, 10% waste. Area: 30 × 20 = 600 sq ft. Bag coverage at 12 in: 48 × 6 ÷ 12 = 24 sq ft. Bags exact: 600 ÷ 24 = 25. With waste: 25 × 1.10 = 27.50, rounded up to 28 bags. In practice that means buying 28 bags, and expecting the last ones to go on joists and gaps. If the same attic were insulated 16 in deep instead, the need would rise to 37 bags, but the attic insulated would be identical in size.
Every branch, checked
With a 36 ft long attic instead of 30 ft, the area rises to 720 sq ft and the need to 33 bags. With 16 in deep insulation, the bag coverage falls to 18 sq ft and the need rises to 37 bags. With 8 in deep insulation, the bag coverage rises to 36 sq ft and the need falls to 19 bags. With 5% waste, the exact need stays 25 bags but the buy falls to 27 bags, since 25 × 1.05 is 26.25 and still rounds up to 27. Attic size, depth and waste each move the order on their own, so a wrong depth figure is as costly as a wrong measurement.
The reverses, worked backwards
The length behind 28 bags is 30 ft: at 24 ft the need is 22 bags (24 ft × 20 ft attic, 480 sq ft, 12 in deep, 20 bags exact, 10% waste); at 36 ft it is 33 bags (36 ft × 20 ft attic, 720 sq ft, 12 in deep, 30 bags exact, 10% waste). The depth behind 28 bags is 12 in: at 10 in the need is 23 bags (30 ft × 20 ft attic, 600 sq ft, 10 in deep, 20.83 bags exact, 10% waste); at 16 in it is 37 bags (30 ft × 20 ft attic, 600 sq ft, 16 in deep, 33.33 bags exact, 10% waste). The waste behind 28 bags is 10%: at 0% the need is 25 bags (30 ft × 20 ft attic, 600 sq ft, 12 in deep, 25 bags exact, 0% waste); at 15% it is 29 bags (30 ft × 20 ft attic, 600 sq ft, 12 in deep, 25 bags exact, 15% waste). Each reverse moves one input and holds the rest, so the step is exactly one dimension, depth or waste change.
What the defaults assume
The defaults assume a 30 ft by 20 ft rectangular attic, 12 in of insulation, bags rated 48 sq ft at 6 in depth, and 10% waste for joists and gaps. They do not assume an R-value target, a vapor barrier or ventilation — an attic with a high R-value target counts its depth separately, and the bags follow the depth, not the label.
Who uses this
- A homeowner insulating an attic: the 28-bag order settles the store call before the first bag is opened.
- A DIYer topping up attic insulation: the same maths on the attic alone gives the bags, not a whole house.
- A contractor quoting a job: attic size, depth, waste — the quote follows the same three steps, and the depth line shows the client why.
- A landlord budgeting an energy refit: the attic size off the plans, multiplied out, gives the bag line on the refit budget.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is the depth and the bag label. Real attics have joists, wires, vents and settling, and the label on the bag overrides any planning figure; an attic measured gross over-orders, and a coverage figure read off a flat attic under-orders one cut with joists and vents. Treat 28 bags as the planning order for a 30 ft by 20 ft attic at 12 in deep and 10% waste, and allow 15% for an attic cut with joists and vents.
Cite this calculator
Bright Side Kit, “Attic Insulation Calculator: 28 Bags for a 30x20 ft Attic”, https://brightoolbox.com/attic-insulation-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (30 ft × 20 ft attic, 12 in deep, 10% waste) so the 28-bag result can be checked.
Frequently asked questions
How many bags of insulation do I need for a 30x20 ft attic?
28 bags at 12 in deep and 10% waste. The area is 600 sq ft; at 24 sq ft a bag at 12 in deep that is 25 bags exact, and 25 × 1.10 = 27.50, rounded up to 28.
How do I work out insulation from an attic size?
Multiply length by width for the area, divide by the bag coverage at your depth, add the waste and round up. A 30 ft by 20 ft attic at 12 in is 600 ÷ 24 = 25 bags exact, or 28 bags at 10% waste.
How much waste should I allow for attic insulation?
10% for a plain attic, 15% for one cut with joists, wires and vents. Every gap takes a bag, so the waste moves the order directly.
Does this count the R-value too?
No — this calculator counts bags only, from the depth you enter. The R-value follows the depth and the material, and is planned separately, by the attic.
Should I buy the exact bags or round up?
Round up: bags are sold by the bag, and the spare bags are the bags the joists and gaps consume. An attic insulated to the exact bag runs short before the last bay.
What if my attic is not a rectangle?
Work out each section separately, add the areas, and divide by the coverage. The square footage calculator handles the area step section by section.