Insulation Calculator

This insulation calculator works out how much insulation you need to reach a target R-value in an attic or wall. For blown-in cellulose or fiberglass it gives the depth to install and the number of bags. For batts or rolls it turns your area and the coverage printed on the package into a bag count.

It is built for homeowners topping up an attic or insulating a new space. You can subtract the R-value of insulation that is already there, change the R-value per inch to match your product, and check the DOE and ENERGY STAR attic recommendations for your climate zone in the table under the calculator.

How to Use the Insulation Calculator

1
Enter the area
Choose US or metric units, then enter the length and width of the attic floor or wall, or type the total area directly.
2
Pick a target R-value
Select a common R-value from the list or enter a custom one. Use the climate zone table below the calculator as a guide for attics.
3
Choose blown-in or batts
For blown-in, pick cellulose or fiberglass (or enter your own R per inch) and the existing R-value. For batts, enter the coverage per bag from the package and a waste allowance.
4
Add your bag coverage
For blown-in, type the coverage per bag from the bag chart for the R-value you are adding. Leave it blank to see a rough estimate instead.
5
Read the result
Blown-in shows the settled depth to install and the bags needed. Batts show the bags needed including waste.

How the insulation calculator works

R-value measures resistance to heat flow, and for loose-fill insulation it rises roughly in proportion to depth. The calculator subtracts any existing R-value from your target and divides the difference by the R-value per inch of the material. It uses about R-2.5 per inch for loose-fill fiberglass and about R-3.5 per inch for cellulose, which are typical published figures. Real products vary, so you can edit the value.

Bag counts come from coverage. In the US, loose-fill bags carry a coverage chart listing, for each R-value, the minimum settled thickness and how many square feet a bag covers. Enter that coverage and the calculator divides your area by it. If you leave coverage blank, it estimates it by assuming a bag covers about 200 sq ft at 1 inch deep for cellulose or 600 sq ft at 1 inch for fiberglass, divided by the depth. Treat that estimate as rough.

For batts, the calculator multiplies the area by 1 plus your waste percentage and divides by the coverage per package, then rounds up.

Blown-in depth (in) = (target R − existing R) ÷ R per inch
Blown-in bags = area ÷ coverage per bag (round up)
Batt bags = area × (1 + waste %) ÷ coverage per bag (round up)
RSI = R × 0.1761

Worked example: a 30 x 40 ft attic to R-49

The attic floor is 30 x 40 ft, which is 1,200 sq ft, and has no insulation. The target is R-49.

Cellulose at R-3.5 per inch: 49 ÷ 3.5 = 14 inches settled depth.
Estimated coverage: 200 ÷ 14 = about 14.3 sq ft per bag, so 1,200 ÷ 14.3 = 84 bags.
Fiberglass at R-2.5 per inch: 49 ÷ 2.5 = 19.6 inches settled depth.
Estimated coverage: 600 ÷ 19.6 = about 30.6 sq ft per bag, so 1,200 ÷ 30.6 = 40 bags.
Topping up instead: with existing insulation of about R-11, you add R-38. With cellulose that is 38 ÷ 3.5 = 10.9 inches and roughly 66 bags by the estimate.

Depth needed by R-value

Settled depth for common attic targets, starting from a bare attic, using the calculator's default R-values per inch. Your product's bag chart is the final word.

Target	Fiberglass (R-2.5/in)	Cellulose (R-3.5/in)	RSI
R-30	12 in	8.6 in	5.28
R-38	15.2 in	10.9 in	6.69
R-49	19.6 in	14 in	8.63
R-60	24 in	17.1 in	10.57

Recommended attic R-values by climate zone

These are the DOE and ENERGY STAR recommendations for attics. The first column is for an attic with no insulation. The second is how much to add if you already have about 3 to 4 inches. Zone 1 covers the warmest parts of the US and zones 5 to 8 the coldest.

Climate zone	Uninsulated attic	Add to 3 to 4 in existing
1	R-30 to R-49	R-25 to R-30
2	R-30 to R-60	R-25 to R-38
3	R-30 to R-60	R-25 to R-38
4	R-38 to R-60	R-38
5 to 8	R-49 to R-60	R-38 to R-49

Tips and common mistakes

Seal air leaks around top plates, pipes, wiring and the attic hatch before adding insulation. Insulation slows heat flow but does not stop air leaking through it.
Install baffles at the eaves so insulation does not block soffit vents.
Use the bag chart, not the estimate, when you buy. Coverage depends on the product and how densely it is blown.
Fit depth markers (attic rulers) to joists before you start so you can see when you reach the target depth.
Batt coverage already assumes standard 16 or 24 inch framing. Compressing a batt into a shallower cavity lowers its R-value.

Safety and code notes

Wear a respirator or dust mask, goggles, gloves and long sleeves, and step only on joists or boards. Keep insulation away from recessed lights that are not rated for insulation contact, and from chimneys and flues, following the product label and local code for clearances. If your attic has old vermiculite (a pebbly, gray-brown or silvery material), leave it undisturbed and have it tested, since some vermiculite contains asbestos. Your local building code sets the minimum R-values for new work, and it takes priority over any estimate here.

Features

Blown-in mode: depth from target R-value minus existing R-value
Editable R-value per inch (cellulose about R-3.5, fiberglass about R-2.5)
Bag count from your bag chart coverage, or a rough estimate if left blank
Batt mode: bags from coverage per package plus a waste allowance
DOE / ENERGY STAR attic R-value table by climate zone
US (sq ft, inches) and metric (m², cm) units with RSI shown

Frequently Asked Questions

How much blown-in insulation do I need for R-49?

At about R-3.5 per inch, cellulose needs 14 inches of settled depth for R-49 in a bare attic. Fiberglass at about R-2.5 per inch needs 19.6 inches. For bag counts, read the coverage chart on the bag at R-49 and divide your attic area by it. As a rough estimate, a 1,200 sq ft attic takes about 84 bags of cellulose or 40 of fiberglass.

What R-value should my attic be?

DOE and ENERGY STAR recommend R-30 to R-49 for an uninsulated attic in zone 1, R-30 to R-60 in zones 2 and 3, R-38 to R-60 in zone 4, and R-49 to R-60 in zones 5 to 8. If you already have 3 to 4 inches, add R-25 to R-49 depending on zone. Check your local building code for the legal minimum.

How many inches of insulation is R-38?

It depends on the material. Loose-fill fiberglass at about R-2.5 per inch needs about 15.2 inches for R-38, while cellulose at about R-3.5 per inch needs about 10.9 inches. Fiberglass batts rated R-38 are around 10 to 12 inches thick. Always follow the minimum settled thickness printed on the bag chart for your product.

Is cellulose or fiberglass better for attic insulation?

Both reach the same R-value if installed to the right depth. Cellulose has a higher R-value per inch, about R-3.5 compared with about R-2.5 for loose-fill fiberglass, so it needs less depth, but it is heavier per square foot and settles more. Fiberglass is lighter and does not absorb moisture as readily. The calculator lets you compare depth and bags for both.

Can I add new insulation on top of old insulation?

Usually yes, as long as the old insulation is dry and free of mold or pests. Enter its approximate R-value as the existing R-value, and the calculator works out only the depth you need to add. Do not add a faced batt on top of existing insulation, since the facing can trap moisture. If you suspect vermiculite, have it tested before disturbing it.

How do I calculate how many bags of batt insulation I need?

Multiply the area by 1 plus your waste allowance and divide by the square feet each package covers, then round up. For a 1,200 sq ft attic floor with packages covering 40 sq ft and 5% waste, that is 1,200 x 1.05 ÷ 40 = 31.5, so 32 bags. Package coverage varies by R-value and batt width, so use the number printed on your product.

Last reviewed September 21, 2026