Insulation Coverage Chart: Bags Needed by Area & R-Value

What does 9 bags mean?

It means 1,000 sq ft of area, plus 10% waste, needs 1,100 sq ft of insulation, and at 128 sq ft a bag that is 8.59 bags — rounded up to 9, because bags are bought whole. The two numbers people confuse are the exact count and the order count: 8.59 tells you how close you are to the next bag, and 9 is what you buy.

Coverage table: bags by area at 128 sq ft a bag, 10% waste

Bag sizes on this page: verified coverage figures

The default coverage, 128 sq ft a bag, is the coverage printed for Owens Corning EcoTouch R-19 unfaced fiberglass batt, 24 inches wide by 48 inches long, in the maker’s product data sheet. The R-38 row below is from the same maker’s product data sheet for its R-38 faced batt, 16 inches by 48 inches, at 42.67 sq ft a bag. Coverage varies by width, length and R-value, so if your bag prints a different figure, type that figure in — the calculator does the rest.

Three things to check before you trust the number

Is the coverage figure from your bag? A bag that covers 42.67 sq ft needs 26 bags for 1,000 sq ft at 10% waste, while a bag that covers 128 sq ft needs only 9. The coverage input is the whole calculation — a wrong coverage gives a wrong order, however careful the area measurement is.

Is the area the area to insulate, not the floor area? Measure each surface you will actually cover — wall, attic floor or floor over unheated space — and add them. A house with 1,000 sq ft of floor can easily have more than 1,000 sq ft of surface to insulate once walls are counted.

Is 10% waste right for your job? Simple rectangles with few obstacles can use less; cut-up areas, many windows, pipes and wiring runs need more. The waste percentage is an input for exactly this reason — set it to your own allowance rather than trusting the default blindly.

Worked example

On the defaults: 1,000 sq ft, 128 sq ft a bag, 10% waste.

1. The adjusted area: 1,000 × (1 + 10 ÷ 100) is 1,100.00 sq ft.
2. The exact bags: 1,100.00 ÷ 128.00 is 8.59 bags.
3. The bags to order: rounding 8.59 up gives 9 bags.

And the other way: the area covered by 10 bags at 128 sq ft a bag with 10% waste is 1,163.64 sq ft, the coverage per bag needed for 1,000 sq ft in 8 bags with 10% waste is 137.50 sq ft a bag, and the largest area (step 10 sq ft) covered by 9 bags at 128 sq ft a bag with 10% waste is 1,040 sq ft.

Every branch, checked

• 500 sq ft at 128 sq ft a bag, 10% waste: 500 sq ft at 128 sq ft a bag, 10% waste: 5 bags to order (4.30 exact) for 500 sq ft at 128.00 sq ft a bag with 10.00% waste — 550.00 sq ft adjusted.
• 2,000 sq ft at 128 sq ft a bag, 10% waste: 2,000 sq ft at 128 sq ft a bag, 10% waste: 18 bags to order (17.19 exact) for 2,000 sq ft at 128.00 sq ft a bag with 10.00% waste — 2,200.00 sq ft adjusted.
• 1,000 sq ft at 42.67 sq ft a bag, 10% waste: 1,000 sq ft at 42.67 sq ft a bag, 10% waste: 26 bags to order (25.78 exact) for 1,000 sq ft at 42.67 sq ft a bag with 10.00% waste — 1,100.00 sq ft adjusted.
• 1,000 sq ft at 77.5 sq ft a bag, 10% waste: 1,000 sq ft at 77.5 sq ft a bag, 10% waste: 15 bags to order (14.19 exact) for 1,000 sq ft at 77.50 sq ft a bag with 10.00% waste — 1,100.00 sq ft adjusted.
• 1,500 sq ft at 128 sq ft a bag, 0% waste: 1,500 sq ft at 128 sq ft a bag, 0% waste: 12 bags to order (11.72 exact) for 1,500 sq ft at 128.00 sq ft a bag with 0.00% waste — 1,500.00 sq ft adjusted.
• 800 sq ft at 128 sq ft a bag, 10% waste: 800 sq ft at 128 sq ft a bag, 10% waste: 7 bags to order (6.88 exact) for 800 sq ft at 128.00 sq ft a bag with 10.00% waste — 880.00 sq ft adjusted.

The reverses, worked backwards

• Area covered by 10 bags at 128 sq ft a bag with 10% waste: 1,163.64 sq ft — at 1,163.64 sq ft the figures are 10 bags to order (10.00 exact) for 1,163.64 sq ft at 128.00 sq ft a bag with 10.00% waste — 1,280.00 sq ft adjusted.; at the lower neighbor the figures are 10 bags to order (9.91 exact) for 1,153.64 sq ft at 128.00 sq ft a bag with 10.00% waste — 1,269.00 sq ft adjusted.; at the upper neighbor the figures are 11 bags to order (10.09 exact) for 1,173.64 sq ft at 128.00 sq ft a bag with 10.00% waste — 1,291.00 sq ft adjusted.. — verdict: STEP.
• Coverage per bag needed for 1,000 sq ft in 8 bags with 10% waste: 137.50 sq ft a bag — at 137.50 sq ft a bag the figures are 8 bags to order (8.00 exact) for 1,000 sq ft at 137.50 sq ft a bag with 10.00% waste — 1,100.00 sq ft adjusted.; at the lower neighbor the figures are 9 bags to order (8.06 exact) for 1,000 sq ft at 136.50 sq ft a bag with 10.00% waste — 1,100.00 sq ft adjusted.; at the upper neighbor the figures are 8 bags to order (7.94 exact) for 1,000 sq ft at 138.50 sq ft a bag with 10.00% waste — 1,100.00 sq ft adjusted.. — verdict: STEP.
• Largest area (step 10 sq ft) covered by 9 bags at 128 sq ft a bag with 10% waste: 1,040 sq ft — at 1,040 sq ft the figures are 9 bags to order (8.94 exact) for 1,040 sq ft at 128.00 sq ft a bag with 10.00% waste — 1,144.00 sq ft adjusted.; at the lower neighbor the figures are 9 bags to order (8.85 exact) for 1,030 sq ft at 128.00 sq ft a bag with 10.00% waste — 1,133.00 sq ft adjusted.; at the upper neighbor the figures are 10 bags to order (9.02 exact) for 1,050 sq ft at 128.00 sq ft a bag with 10.00% waste — 1,155.00 sq ft adjusted.. — verdict: STEP.

What the defaults assume

The defaults are 1,000 sq ft at 128 sq ft a bag with 10% waste, so the headline is 9 bags to order from 8.59 exact. The model is one multiplication for the waste-adjusted area and one division for the bag count, rounded up to whole bags. The area reverse is checked at 10 sq ft either side, because 10 sq ft at 10% waste is 11 sq ft of adjusted area — about 0.09 of a bag — and shows the step cleanly at the 10-to-11 boundary; the coverage reverse is checked at 1 sq ft a bag either side, because 1 sq ft a bag moves the exact count by about 0.06 of a bag near 8 bags. The model does not count compression, settling, double layers, or the R-value your climate zone requires — those belong to the product and the building code, not to the bag-count arithmetic.

Output · formula · meaning

Who uses this

• A homeowner insulating an attic — 1,000 sq ft at 128 sq ft a bag is 9 bags with 10% waste: the shopping list before the store trip.
• A contractor quoting a job — the coverage table gives the bag count for every area from 100 to 2,000 sq ft at a glance, and the R-38 row shows why a thicker batt changes the order so much.
• A DIYer comparing two products — the same 1,000 sq ft needs 9 bags at 128 sq ft a bag but 26 bags at 42.67 sq ft a bag: coverage per bag is the price comparison, not the bag price alone.
• A student learning division on a real job — one worked example from area to adjusted area to exact bags to whole bags, and the branches that show how each input moves the answer.

Accuracy & limitations

Figures are the model’s own arithmetic on the numbers you type, computed from unrounded values and displayed rounded HALF_UP to 2dp, with bag counts rounded UP to whole bags. The main limitation is the input, not the arithmetic: coverage per bag varies by product, and the two figures on this page are verified only for the two Owens Corning EcoTouch products named — any other product must use the coverage printed on its own bag. The model also assumes batts laid flat at full coverage with no compression or settling loss, and it does not check whether the R-value suits your climate zone or building code. This page is a calculation tool, not building or installation advice.

Cite this calculator

1,000 sq ft at 128 sq ft a bag needs 9 bags with 10% waste. Source: Bright Side Kit, brightoolbox.com. Bag coverage: Owens Corning EcoTouch product data sheets (R-19 unfaced 24 in × 48 in at 128 sq ft a bag; R-38 faced 16 in × 48 in at 42.67 sq ft a bag).

Frequently asked questions