Insulation Calculator
Enter an attic or wall area, the R-value you want and any insulation already there to get the R still needed and the bags of blown-in insulation or batts to buy, rounded up once.
Estimates for planning, not professional advice. Read the full notice.
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Attic floor: shape and size
Skip the drawingDrag a round corner to move it. The small hollow diamond on a side is its curve handle: drag it across the side to curve the side (it is not a corner). To add a corner (a new point), tap the side itself, then choose Add a point here. Tapping a corner or a side opens its actions. Drag empty space to move the view; pinch, or Ctrl and the mouse wheel, to zoom. The keys are listed under Keys.
Keys
- Tab
- next corner; each says how far it is from corner 1
- Arrow keys on a corner
- move it one square; ten with Shift; 1/16 in with Alt
- Delete or Backspace
- remove the corner; its two sides join in one straight side
- + on a corner
- add a corner halfway along the next side
- Arrow keys on the drawing
- move the view; farther with Shift
- + and - on the drawing
- zoom in and out
- 0 on the drawing
- show the whole shape
- Ctrl+Z (Cmd+Z on a Mac)
- undo, except inside a typed field (it keeps its own)
- Ctrl+Shift+Z or Ctrl+Y
- redo
- Escape while dragging
- put it back where it was
- Enter on a corner or side
- go to its actions
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- Area
- 1,000 ft²
- Perimeter
- 130 ft
- Cut-out edges
No area until the shape is fixed.
Estimate for planning, from what you entered and the product data shown. Check product labels and your supplier before you buy.
Shape: Rectangle Change
Sides
Corners
Cut-outs
Feet and inches, like 12' 6"
Example: an attic with none yet
26 bags of blown-in fiberglass
Your project
Enter a size to see your total.
Each product is rounded up once for the whole project, so the total can be less than the items bought alone added together. A line counted from a cut layout (sheets, boards or tiles laid out piece by piece) is counted for each item and added.
Shopping list
| Item | Size | Buy |
|---|---|---|
| Blown-in fiberglass bags | bags of blown-in fiberglass | 26 |
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Worked example: an attic with none yet
Show the working
- R still needed: the target minus the R already there 49 R − 0 R = 49 R
- Pick the smallest chart row at or above that R 49 R = 49 R row
- Bags: the area over the maximum net coverage of one bag, which gives more bags than the bags per thousand 1,000 ft² ÷ 39.9 ft²[1] = 25.06 bags
- Round up once, to whole bags 25.06 bags = 26 bags
Assumptions
The figures behind the example. Sourced figures link to their source below; a default is a starting value to check against your label or supplier.
| Assumption | Value | Details and changes |
|---|---|---|
| Batt size | No default | |
| Default The width and length printed on your batt. They are fields above the result. Check your label or supplier. | ||
Feet and inches, like 12' 6"
Bags and settled depth by R-value row
Bags per 1,000 ft², maximum net coverage of one bag and minimum settled thickness for each R-value row of one maker's blown-in fiberglass chart
| R-value row | Bags per 1,000 ft² | Maximum net coverage of one bag | Minimum settled thickness |
|---|---|---|---|
| R-13 | 5.9 bags | 169.8 ft² | 5 in |
| R-19 | 9 bags | 110.7 ft² | 7 in |
| R-22 | 10.5 bags | 94.8 ft² | 8 in |
| R-26 | 12.6 bags | 79.3 ft² | 9.5 in |
| R-30 | 14.6 bags | 68.5 ft² | 10.75 in |
| R-38 | 19 bags | 52.6 ft² | 13.5 in |
| R-44 | 22.4 bags | 44.6 ft² | 15.5 in |
| R-49 | 25 bags | 39.9 ft² | 17 in |
| R-60 | 31.5 bags | 31.8 ft² | 20.5 in |
Figures in the Bags per 1,000 ft², Maximum net coverage of one bag and Minimum settled thickness columns are per Owens Corning's product page, checked Nov 12, 2026.
Product names are trademarks of their owners. HoneyDoMath is not affiliated with, sponsored by or endorsed by these makers. Figures come from each maker's published sheets and are shown for information only.
Source: [1]
The calculator uses the smallest row whose R-value reaches the R still needed. It never mixes two rows, and it does not interpolate between them. The chart prints two figures for each row, bags per 1,000 ft² and the coverage of one bag; where they differ the calculator uses the one that gives more bags.
Questions
How much insulation do I need for my attic?
Enter the attic's length and width, choose the R-value you want and, if there is insulation already, its depth and kind. The calculator takes the R already there off your target, finds the chart row that reaches the rest and counts the bags. The area counts gross, with no deduction for framing.
What R-value does the insulation already there give?
The depth in inches times the R per inch of its kind, using the figures ENERGY STAR prints for loose fiberglass, rock wool, cellulose and fiberglass batts. A depth of zero gives zero R.
How many bags or batts do I need?
For blown-in bags the count is the area in thousands of square feet times the bags per thousand square feet on the chart row. For batts it is the area divided by the area of one batt, rounded up once. Enter your batt's width and length from its label.
What if my target is lower than what is already there?
Then nothing is needed and the calculator says so with zero to buy. A target above the highest row of the chart cannot be counted from it, and the calculator asks you to choose a lower target.
Sources (2)
-
AttiCat® PINK® Blown-In Insulation, technical information coverage table (opens in new tab), Owens Corning Sales, LLC, product page, undated.
Manufacturer data.
“R-VALUE BAGS PER 1000 SQ. FT. MAXIMUM NET COVERAGE MINIMUM WEIGHT / SQ. FT. MINIMUM THICKNESS (IN) … R-44 22.4 44.6 0.616 15.5 15.5 R-49 25 39.9 0.688 17 17 R-60 31.5 31.8 0.866 20.5 20.5”
“R-VALUE BAGS PER 1000 SQ. FT. MAXIMUM NET COVERAGE MINIMUM WEIGHT / SQ. FT. MINIMUM THICKNESS (IN) … R-13 5.9 169.8 0.162 5 5 R-19 9 110.7 0.248 7 7 R-22 10.5 94.8 0.29 8 8”
“R-VALUE BAGS PER 1000 SQ. FT. MAXIMUM NET COVERAGE MINIMUM WEIGHT / SQ. FT. MINIMUM THICKNESS (IN) … R-26 12.6 79.3 0.347 9.5 9.5 R-30 14.6 68.5 0.402 10.75 10.75 R-38 19 52.6 0.523 13.5 13.5”
Per Owens Corning's product page, checked Nov 12, 2026. Figures and codes change; verify with the source. Back to where this is used (Source 1) -
What to Look For: DIY Checks and Inspections (opens in new tab), ENERGY STAR (U.S. Environmental Protection Agency and Department of Energy), web page, undated.
Code or government.
“Loose fibers: light-weight yellow, pink, or white fiberglass =2.5 x depth dense gray or near white, may have black specs rock wool =2.8 x depth small gray flat pieces or fibers (from newsprint) cellulose =3.7 x depth”
“Batts: light-weight yellow, pink, or white fiberglass =3.2 x depth”
Checked November 12, 2026. Figures and codes change; verify with the source.
These are summaries of what the sources say, not professional advice. Codes and product data change. Check with your building department and the product label about your own project.
Sources
- [1] Owens Corning, AttiCat® PINK® Blown-In Insulation, technical information coverage table. Checked November 12, 2026. https://www.owenscorning.com/en-us/insulation/products/atticat
- [2] ENERGY STAR (U.S. Environmental Protection Agency and Department of Energy), What to Look For: DIY Checks and Inspections. Checked November 12, 2026. https://www.energystar.gov/saveathome/seal_insulate/identify-problems-you-want-fix/diy-checks-inspections