Blown-In Insulation Calculator

How many bags of blown-in insulation do you need?

Divide your attic floor area by the square feet one bag covers. This calculator uses about 40 sq ft per bag at R-30, which matches common 25 lb loose-fill cellulose bags. Loose-fill fiberglass bags usually cover more, often around 55 to 65 sq ft at R-30, so check the chart on your bag. A 30 × 20 ft attic is 600 sq ft, so 600 ÷ 40 = 15 bags. At $14 a bag, that is $210.

Bags are sold whole, so round any fraction up. The calculator above is set for R-30. For a higher R-value, scale the bag count with the table below.

Bags needed by R-value

A thicker layer holds more R-value, so each bag covers less floor. These counts scale the R-30 figure of 40 sq ft per bag. They are planning numbers.

Target R-value	Bags per 1,000 sq ft	Bags for 600 sq ft	Bags for 1,176 sq ft
R-19	about 16	10	19
R-30	25	15	30
R-38	about 32	19	38
R-49	about 41	25	48
R-60	50	30	59

Real bags differ. Brands and bag weights have different coverage charts, and some bags cover more than 40 sq ft at R-30. Check the chart printed on your bag, then adjust in proportion. If the chart says 50 sq ft per bag, a 600 sq ft attic needs 12 bags.

Depth needed by R-value

R-value comes from depth. Cellulose gives roughly R-3.2 to R-3.8 per inch. Loose-fill fiberglass gives roughly R-2.2 to R-2.9 per inch. The depths below use typical mid values and are rounded.

Target	Cellulose (about R-3.5/in)	Fiberglass (about R-2.5/in)
R-19	about 5.5 in	about 7.5 in
R-30	about 8.5 in	about 12 in
R-38	about 11 in	about 15 in
R-49	about 14 in	about 19.5 in
R-60	about 17 in	about 24 in

The bag chart lists the minimum settled thickness for your exact product. It wins over this table.

Cellulose vs. fiberglass

Cellulose is denser and gives more R-value per inch. It settles more over time. Fiberglass is lighter, does not absorb as much moisture, and needs more depth for the same R-value.

For the same R-value, fiberglass usually needs more inches and a different bag count. Do not mix the two charts. Use the chart for the product you buy.

What R-value do you need?

The U.S. Department of Energy suggests roughly R-30 to R-49 for attics in the hottest regions. It suggests about R-49 to R-60 in most of the rest of the country, including cold climates. The exact target depends on your climate zone and local code. Check your local code before you buy.

If you already have insulation, measure its depth. Dry, clean old insulation still counts. Add only the difference.

Worked examples

Example 1: small ranch attic, 30 × 20 ft, $14 per bag.

• Area: 30 × 20 = 600 sq ft
• Bags: 600 ÷ 40 = 15 bags
• Cost: 15 × $14 = $210

Example 2: bonus attic over a two-car garage, 42 × 28 ft, $14 per bag.

• Area: 42 × 28 = 1,176 sq ft
• Bags: 1,176 ÷ 40 = 29.4, rounded up to 30 bags
• Cost: 30 × $14 = $420

Example 3: the same 1,176 sq ft attic at R-49.

• Scale factor: 49 ÷ 30 = 1.63
• Bags: 30 × 1.63 = about 48 bags
• Cost: 48 × $14 = about $672

Confirm each result against the chart on your bag.

How to measure your attic

1. Measure the attic floor, not the roof. Insulation sits on the ceiling below, between and over the joists.
2. Split odd shapes. An L-shaped house is two rectangles. Run each one and add the bags.
3. Leave out areas you will not fill, such as floored storage sections.
4. Mark recessed lights and chimneys. Keep the required clearance around them.

Need the floor area first? Try the square footage calculator.

Before you blow

Staple a few rulers to the joists and mark your target depth. They show whether the layer is even. Without them you get a mound near the hatch and a thin layer at the far eaves.

Many stores let you return unopened bags. Buy one or two extra.

Common mistakes

• Skipping air sealing. Open top plates, attic hatches, and plumbing chases leak warm air through new insulation. Seal them first.
• Blocking soffit vents. Fit baffles at the eaves so loose fill cannot choke the airflow.
• Buying for the wrong R-value. Buying 15 bags for an R-49 attic leaves you about 40% short.
• Filling to the settled depth. Cellulose settles after install. The bag lists both installed and settled thickness, so fill to the installed figure.
• Covering non-IC rated light fixtures. Older recessed cans can overheat. Box them or swap them for IC-rated units.
• Pushing insulation over damp or moldy material. Fix the leak and remove wet insulation first.

Related calculators

For batts in walls and open joists, use the insulation calculator. The spray foam insulation cost calculator prices a sealed attic. The heat loss calculator shows what extra R-value saves. If you are also working on ventilation at the eaves, see the soffit calculator.