How It Works
Required depth = target R-value ÷ the material’s R-value per inch (e.g. R-38 ÷ 2.5 for blown fiberglass = 15.2 in).
Depth (in) = Target R ÷ R-per-inch | Loose-fill bags = ⌈Area × Depth ÷ 12 ÷ bag coverage⌉ | Batts/board packages = ⌈Area ÷ package coverage⌉ | Spray foam = Area × Depth (board-feet)
- For loose-fill (blown fiberglass, cellulose, or mineral wool): cubic feet = area × depth ÷ 12, then bags = cubic feet ÷ your coverage-per-bag figure, rounded up.
- For batts, rolls, or rigid board: packages = area ÷ your coverage-per-package figure, rounded up — these are fixed-thickness products, so coverage is an area, not a volume.
- For spray foam (open- or closed-cell): the quantity is board-feet = area × depth, because foam is sold and quoted by the board-foot, not by the bag.
- R-value achieved = depth × R-per-inch, confirming you hit the target. Estimated cost = quantity needed × the price you enter.
Worked Example
1,000 sq ft attic, R-38 target, blown fiberglass (R-2.5/in), 40 cu ft per bag, $15/bag.
Required depth
38 ÷ 2.5 = 15.2 in
Cubic feet
1,000 × 15.2 ÷ 12 = 1,267 cu ft
Bags (at 40 cu ft/bag)
⌈1,267 ÷ 40⌉ = 32 bags
Estimated cost
32 × $15 = $480
Same attic as fiberglass batt (R-3.2/in), 88 sq ft/package
38 ÷ 3.2 = 11.9 in → ⌈1,000 ÷ 88⌉ = 12 packages
Same attic in open-cell spray foam
38 ÷ 3.7 = 10.3 in → 10,271 board-feet
R-38 in a 1,000 sq ft attic takes about 32 bags of blown fiberglass at 15.2 inches deep (≈$480 in material at $15/bag) — at the 40 cu ft per bag this example assumes; always use the coverage chart on the bag you buy. The same attic in a fiberglass batt product would need about 12 packages at that product’s 88 sq ft/package coverage, and in open-cell spray foam is quoted as ~10,271 board-feet (1 board-foot = 1 sq ft at 1 in) — three different ways the same R-38 target gets bought and priced.
How to Estimate Insulation: Depth, Bags, and R-Value
What this calculator estimates and who it is for
This tool works backward from the R-value you want to the practical numbers you need: how deep the insulation layer has to be, and how much material that depth and your area add up to — bags for loose-fill, packages for batts/rolls/rigid board, or board-feet for spray foam. It also reports the R-value you actually achieve at the calculated depth, and an optional material cost, so you can confirm you have hit the target and get a rough budget in one pass.
It is aimed at homeowners adding insulation to an attic, wall, ceiling, or floor, and at anyone pricing a job before buying material or renting a blower — whether that’s loose-fill fiberglass, cellulose, or mineral wool, fiberglass or mineral-wool batts and rolls, rigid foam board, or spray foam.
How the calculation works
Required depth is the target R-value divided by the material’s R-value per inch — a higher R-per-inch product such as closed-cell foam or a rockwool batt reaches the same R-value in far less thickness than blown fiberglass. That depth is multiplied by the area to get the volume or coverage needed.
For loose-fill (blown fiberglass, cellulose, or mineral wool), the bag count divides the cubic feet needed by the coverage-per-bag figure you enter from the product chart and rounds up. For batts, rolls, or rigid board, the package count instead divides the area directly by the package’s coverage area, since those are fixed-thickness products rather than a variable-depth fill. For spray foam, the quantity is reported in board-feet — area times depth in inches — because foam is sold and quoted that way. The R-value achieved is simply the calculated depth times the R-per-inch, which should match your target once the depth is set, and the cost estimate is the quantity times whatever price you enter.
Reading the result and ordering material
Use the depth figure to mark your target on the rafters or a depth gauge so you blow evenly to the right thickness everywhere — coverage falls short in the corners and eaves if you do not watch the depth. Order the bag count, then add a spare bag or two so you are not stopped just short of the far corner of an attic.
Always cross-check the bag count against the coverage chart on the product you actually buy. Manufacturers print bags-per-area at each R-value, and that chart, tuned to the real settled density, is more reliable than any generic per-bag figure.
Common mistakes
The biggest planning error is ignoring insulation that is already in place and ordering for the full target rather than the gap. Measure what is there, convert it to an R-value, and subtract it first. The second is comparing materials by depth instead of R-value — three inches of one product is not three inches of another.
People also blow attics short in the eaves where the roof meets the wall, leaving a cold ring around the edge, and they sometimes block soffit vents in the process. Use baffles to keep airflow paths open while still hitting full depth.
Practical tips
Air-seal before you insulate: caulk and foam the gaps, top plates, and penetrations, because insulation slows heat flow but does not stop air leaks, and a leaky attic wastes the R-value you paid for. Install rafter baffles at the eaves so soffit ventilation keeps working after the insulation goes in.
When you rent a blower, have a helper feed bags while you run the hose, and check the depth with a ruler in several spots as you go. Many stores lend the machine free with a minimum bag purchase, which makes blown fiberglass and cellulose genuinely DIY-friendly.
Limitations and when to confirm with a pro
This estimator covers loose-fill by the bag, batts/rolls/rigid board by the package, and spray foam by the board-foot, and assumes a bare area. It does not account for thermal bridging through framing, or design a vapor or air-barrier strategy — and spray foam, while quantified here, needs professional equipment to install. It also does not estimate energy-bill savings, since that depends on climate, current insulation, and HVAC efficiency in ways a general calculator cannot responsibly model.
Required R-values are set by climate zone and application in the energy code, and crawl spaces and cathedral ceilings have moisture rules of their own. Confirm the target R-value and any vapor-barrier requirements with the IECC or a local building official, and consult an insulation pro for spray foam or moisture-prone assemblies.