Solve backwards
How much attic will my bags of blown-in insulation cover?
- 20 bagsThe example worked below
- Blown-In Insulation Bag Count CalculatorEvery figure is its own
You have
How much blown-in insulation?
Worked by the Blown-In Insulation Bag Count Calculator at the settings below.
The settings it uses
- Target Installed Depth
- 11.8 in
- Bag Coverage Yield at This Depth (m³ per bag)
- 2.6
What it will do
1,875 sq ft of attic floor
174 m²
One attic floor. Enough for a attic floor about 43 × 43 ft.
Or several. About 1 attic floor of the size the Blown-In Insulation Bag Count Calculator opens on: a attic floor about 33 × 33 ft.
Check it forwards
- Area to Insulate
- 1,875 sq ft
- It asks for
- 20 bags
- You have
- 20 bags
The Blown-In Insulation Bag Count Calculator asks for 20 bags at that size, and 21 bags one step larger.
If you had more, or less
Around your amount
Each worked by the Blown-In Insulation Bag Count Calculator at the same settings.
Half
10 bags
938 sq ft
a attic floor about 31 × 31 ft
Yours
20 bags
1,875 sq ft
a attic floor about 43 × 43 ft
Half as much again
30 bags
2,813 sq ft
a attic floor about 53 × 53 ft
Double
40 bags
3,751 sq ft
a attic floor about 61 × 61 ft
Common amounts
What each amount of blown-in insulation covers
The sizes it is usually sold in, each worked by the Blown-In Insulation Bag Count Calculator at the settings above.
| You have | It covers | As one attic floor | Average attic floors |
|---|---|---|---|
| 5 bags | 469 sq ft(43.6 m²) | a attic floor about 21.7 × 21.7 ft | under 1 |
| 10 bags | 938 sq ft(87.1 m²) | a attic floor about 31 × 31 ft | under 1 |
| 20 bags | 1,875 sq ft(174 m²) | a attic floor about 43 × 43 ft | 1 |
| 40 bags | 3,751 sq ft(348 m²) | a attic floor about 61 × 61 ft | 3 |
| 60 bags | 5,626 sq ft(523 m²) | a attic floor about 75 × 75 ft | 5 |
Depth on a ruler is not R-value
A coverage chart ties each R-value to a minimum number of bags per unit of area as well as to a depth. A yield read from a lower-density line gives fewer bags at the same finished depth — the attic measures right and performs below its rating, and nothing on site catches it afterwards. Read the bag's chart at your target R-value.
Yield falls as the depth rises
The calculator opens at 300 mm installed, about 12 inches, and a yield of 2.6 cubic metres a bag at that depth. Coverage per bag decreases as target depth and density rise, so the yield must come from the chart line for your depth, not a generic average — the attic here moves with it.
Attic yields are not wall yields
Dense-packing a wall or a cathedral cavity is done at a far higher density to stop the material settling and leaving a void at the top, which cuts the coverage per bag by half or more — and the fill goes in through a tube around blocking and wiring. These bags covering an attic floor would fill far less wall.
How the answer is worked
Nothing here is a second formula. The Blown-In Insulation Bag Count Calculator runs forwards — a size in, a quantity out — and this page grows its own starting attic floor in proportion until the quantity it asks for is what you have. Where it rounds up to whole containers, the answer is the largest size before the next one would be needed.
Where the calculator adds an allowance for cuts or breakage when you order, the same allowance comes off here, because it is the same calculator: a stack you already own finishes less than its face value. Where it adds none, neither does this page — the settings beside the answer say which.
Blown-In Insulation Bag Count Calculator
runs it forwardsCalculate how many bags of blown-in insulation are needed for an attic or wall cavity at a target depth.
Work out what to buy insteadSolve backwards
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Solve backwards
Start from what is already in the yard
Paint, sheets, rolls, bags and pieces, each read the other way by the calculator that orders it.
The calculator's own figures
Nothing is estimated twice: the page runs the same calculator you would order with.
The largest size that fits
Where a calculator rounds up to whole containers, the answer stops before the next one.
Both unit systems
Gallons and litres, square feet and square metres, side by side.
Frequently asked questions
- How much attic floor does 20 bags of blown-in insulation cover?
- At the Blown-In Insulation Bag Count Calculator's own settings, 1,875 sq ft (174 m²) — a attic floor about 43 × 43 ft. That is about 1 attic floor of the size the calculator opens on: a attic floor about 33 × 33 ft. Every figure follows the settings, which you can change in the calculator.
- How many square feet does a bag of blown-in insulation cover?
- It depends on the target R-value, because the depth and density both rise with it, so one bag covers far more attic at a low R-value than at a high one. The bag's coverage chart gives the figure for each R-value — use that line, not a generic average.
- How deep should blown-in insulation be in an attic?
- Deep enough to reach the R-value your climate zone requires. The bag's chart gives both the installed depth and the minimum number of bags per area for each R-value, and both have to be met — the depth alone does not prove the rating.
- Why does blown insulation need a minimum number of bags?
- Because fluffing the material thinner can reach the right depth with fewer bags and a lower density, which settles and underperforms. The minimum bag count per area on the chart is what guarantees the rated R-value, so count bags as well as measuring depth.