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Insulation Calculator
Use this free insulation calculator to find out how much insulation a wall or attic needs. Enter the area, pick a target R-value from R-13 to R-49, and choose fiberglass batts, blown cellulose, or spray foam. The calculator gives you the depth required and the number of batts, bags, or foam kits to buy, using standard coverage figures stated on the page.
How it works
R-value measures how well insulation resists heat flow, and a bigger number means better insulation. Building codes call for different R-values in different parts of a house: walls are often R-13 or R-19, and attics are often R-30, R-38, or R-49, depending on the climate. This calculator takes your target R-value and your insulation type, works out how thick the insulation has to be, and then tells you how many batts, bags, or foam kits that thickness needs across your area.
Fiberglass batts come in fixed thicknesses matched to standard R-values: R-13 is 3.5 inches thick, R-19 is 6.25 inches, R-30 is 9.5 inches, R-38 is 12 inches, and R-49 is 14 inches. A batt is 93 inches long, so a 15-inch-wide batt covers 9.69 square feet and a 23-inch-wide batt covers 14.85 square feet. The calculator divides your area by the coverage of one batt and rounds up, because a cut batt still counts as a batt used. Match the batt width to your stud spacing: 15-inch batts for studs 16 inches on center, 23-inch batts for studs 24 inches on center.
Blown cellulose is loose fiber blown into an attic or wall cavity, and it gives about R-3.7 per inch. The depth for any target is the R-value divided by 3.7, so R-38 needs 10.27 inches. Coverage per bag comes from the bag itself: a standard 25-pound bag holds about 15.63 cubic feet of settled cellulose at a density of 1.6 pounds per cubic foot, and dividing that volume by the depth gives the area one bag covers. At R-38 that is 18.26 square feet per bag, at R-30 it is 23.13, at R-19 it is 36.51, and at R-49 it is 14.16. The calculator uses these figures and states them here so you can check them against the bag label.
Spray foam is measured in board feet, where one board foot is one square foot at one inch thick. Open-cell foam gives about R-3.6 per inch, so R-38 needs 10.56 inches, while closed-cell foam gives about R-6.5 per inch and reaches R-38 in 5.85 inches. Multiply your area by the depth in inches to get the board feet, then divide by the kit size: a 200-board-foot kit or a 600-board-foot kit. Closed-cell foam costs more per board foot but also seals air and adds strength, while open-cell foam is cheaper and better at filling odd shapes.
Depth is only half the job; coverage is the other half. Gaps, compressed batts, and thin spots all cut the real R-value, sometimes by a lot. A batt squeezed into a cavity thinner than its label thickness loses R-value, and a gap of even a few inches beside a batt lets heat bypass it. In an attic, measure the depth after the insulation settles, and add more if it has dropped below the target. Always wear a mask, gloves, and eye protection when handling insulation, and keep it away from recessed lights unless the lights are rated for contact.
Worked example
An attic floor measures 1,000 square feet and needs R-38 blown cellulose insulation.
Step 1: Find the depth — cellulose gives R-3.7 per inch, so 38 ÷ 3.7 = 10.27 inches deep.
Step 2: Find the coverage per bag — a 25-pound bag holds 15.63 cubic feet, and the depth is 10.27 inches, or 0.856 feet, so one bag covers 15.63 ÷ 0.856 = 18.26 sq ft.
Step 3: Find the bags — 1,000 ÷ 18.26 = 54.77, rounded up to 55 bags.
Step 4: Check the weight — 55 bags × 25 lb = 1,375 lb of cellulose on the attic floor.
The attic needs 55 bags of blown cellulose, installed 10.27 inches deep. For comparison, the same area in fiberglass batts would need 1,000 ÷ 9.69 = 104 batts of 15-inch R-38, and in closed-cell spray foam it would need 1,000 × 5.85 = 5,850 board feet, or 10 kits of 600 board feet.
FAQ
What R-value do I need for an attic?
Most climates call for R-30 to R-49 in an attic, with R-38 as a common target and R-49 in colder regions. Walls are usually R-13 or R-19. Check your local building code, because the required R-value depends on the climate zone your house is in.
How thick is R-38 insulation?
It depends on the material. R-38 fiberglass batts are 12 inches thick, blown cellulose needs 10.27 inches, open-cell spray foam needs 10.56 inches, and closed-cell spray foam needs only 5.85 inches. The calculator shows the depth for the type and R-value you pick.
How many bags of cellulose insulation do I need?
Divide your area by the coverage of one bag at your target R-value. A 25-pound bag covers about 18.26 square feet at R-38, 23.13 at R-30, and 36.51 at R-19. The calculator uses these figures, which come from a settled density of 1.6 pounds per cubic foot.
Are fiberglass batts or blown cellulose better?
Blown cellulose fills gaps and odd shapes better and gives slightly more R-value per inch, while batts are easier to install yourself and easier to remove later. Both work well when installed at the full depth with no gaps. Cellulose is often cheaper for a large attic, and batts suit walls with regular stud spacing.
What is a board foot of spray foam?
One board foot is one square foot of foam at one inch thick. To find the board feet for a job, multiply the area in square feet by the depth in inches. A 1,000-square-foot attic at 5.85 inches deep needs 5,850 board feet of closed-cell foam.