Insulation Calculator
A 20 by 25 foot area at a planning target of R-30 is 2 layers and 20 bags of R-15 batts, or 10.00 inches of blown-in and 42 bags.
Last updated . Length and width start in feet. Switch them to yards or meters. Batt R-15, 50 ft² per bag, 3.0 R per inch, and 10 ft³ per bag are planning figures. No brand claims. No price.
Enter the area and the target R. Batts use the R and the coverage of one layer. Blown-in uses R per inch and the cubic feet on the bag.
The board: drywall calculator. Paint: paint calculator. A roof over the attic: roofing calculator.
Batt and blown-in insulation
R-30 is a planning target for this example, not a code requirement for a city. Both the batt count and the blown-in depth use it.
Batt R-15 and 50 ft² per bag are planning figures for one layer. Blown-in starts at 3.0 R per inch and 10 ft³ per bag so the division can be checked. Cellulose is often nearer 3.5 to 3.7 R per inch, and blown fiberglass nearer 2.2 to 2.5. Read the bag. These are not brand claims. No price.
Results
How the bags are counted
Area is length × width. For batts, layers = target R ÷ batt R, rounded up, and bags = area × layers ÷ square feet per bag, rounded up. R-30 with an R-15 batt is 2 layers. On 500.00 square feet at 50 square feet per bag, that is 20 bags.
For blown-in, depth = target R ÷ R per inch, and volume = area × depth. R-30 at 3.0 R per inch is 10.00 inches, 25.40 cm, and 416.67 cubic feet. At 10 cubic feet per bag that is 42 bags. 3.0 R per inch is a planning start. Cellulose is often nearer 3.5 to 3.7, and blown fiberglass nearer 2.2 to 2.5. None of these numbers is a brand claim.
Worked example
Leave every field as it opens: 20 ft by 25 ft, target R-30, batt R-15, coverage 50 ft², blown-in 3.0 R per inch, yield 10 ft³. Area reads 500.00 ft² · 46.45 m². Batt layers read 2. Batt bags read 20. Blown depth reads 10.00 in · 25.40 cm. Blown volume reads 416.67 ft³ · 11.80 m³. Blown bags read 42.
| Target | Batt | Layers | Batt bags | R / inch | Depth | Blown bags |
|---|---|---|---|---|---|---|
| R-30 | R-15, 50 ft² | 2 | 20 | 3.0 | 10.00 in | 42 |
| R-30 | R-15, 50 ft² | 2 | 20 | 3.5 | 8.57 in | 36 |
| R-38 | R-13, 40 ft² | 3 | 38 | 3.0 | 12.67 in | 53 |
Frequently asked questions
How much insulation for a 20 by 25 attic at R-30?
Leave the area at 20 ft by 25 ft and the target at 30. Area reads 500.00 ft² · 46.45 m². Batt R stays 15 and coverage stays 50, so layers read 2 and batt bags read 20. Blown-in stays at 3.0 R per inch, so depth reads 10.00 in · 25.40 cm. Volume reads 416.67 ft³ · 11.80 m³. At 10 ft³ per bag, blown bags read 42.
What are the batt assumptions?
Layers are the target R divided by the batt R, rounded up. 30 ÷ 15 is 2, so one layer is not enough. Bags are the area times the layers, divided by the square feet on the bag, rounded up. 500 × 2 ÷ 50 is 20. R-15 and 50 ft² are planning figures for one layer, not a brand package.
How deep is blown-in at 3.5 R per inch?
Set blown-in R per inch to 3.5 and leave the target at 30. Depth reads 8.57 in · 21.77 cm. Volume reads 357.14 ft³ · 10.11 m³. Blown bags read 36. Batt layers stay 2 and batt bags stay 20. Cellulose is often nearer 3.5 to 3.7 R per inch, and blown fiberglass nearer 2.2 to 2.5. The 3.0 start is only a planning figure.
What if the target is R-38 and the batt is R-13?
Set the target to 38, the batt R to 13, and the batt coverage to 40. Layers read 3, because 38 ÷ 13 is 2.92 and rounds up. Batt bags read 38. Depth at 3.0 R per inch reads 12.67 in · 32.17 cm. Volume reads 527.78 ft³ · 14.95 m³. Blown bags read 53.
Where does the blown-in volume come from?
Depth in inches is the target R divided by R per inch. Volume is the area times that depth in feet. At R-30 and 3.0 R per inch the depth is 10 inches, and 500 × 10 ÷ 12 is 416.67 cubic feet. Bags divide that volume by the cubic feet on the bag and round up. The 10 ft³ yield is a planning figure. Read the bag.
Does this know my local code R-value?
No. R-30 is the example target so the division can be checked. It is not a code requirement for a city, a climate zone, or a wall versus an attic. Type the R the plan asks for. The same area formula works for a ceiling or a wall once you have the square feet.
Is this drywall or a brand of batt?
No. This page is area, layers or depth, and bags from the rates you type. It does not name a batt, a blow, or a coverage chart. The board over the insulation is the drywall calculator. Paint is the paint calculator.