Insulation Quantity Calculator
Depth for the R-value, and units to buy from the label — because insulation is the rare material whose package does the estimating with you.
The product label is the law here: coverage at a given depth and R-per-inch change by product. Old attics can hide vermiculite or other suspect material — stop and test before disturbing anything that looks like it.
How the math works
Required depth = target R ÷ R-per-inch for your material (blown cellulose ≈ 3.2–3.8, fiberglass batts ≈ 3.1–3.4, mineral wool ≈ 4.0–4.3 — the label on YOUR product is the authority). Units = area ÷ coverage per unit AT THAT DEPTH. Blown-in coverage is depth-dependent, which is why the coverage input says “at that depth” in bold letters everywhere it matters.
A worked example
An R-38 attic in blown cellulose at 3.5 per inch needs 10.86 in of depth. Over 1,000 sq ft at a label coverage of 30 sq ft per bag at that depth, with 5% waste: 35 bags. Change the label coverage and the count moves — that is the product talking, not the calculator guessing.
Questions from the field
How deep is R-38? At 3.5 per inch, 10.86 inches. At 3.2 per inch it is 11.9 — the material decides, which is why R-per-inch is an input.
How many bags of blown insulation do I need? Area ÷ the bag’s coverage at your target depth. Manufacturers print a coverage chart by R-value on every bag; enter the number from the chart row you are blowing to.
Anything to check in an old attic first? Yes: vermiculite and other suspect materials in older homes can contain asbestos. If it looks like grey pebble-like granules, stop, do not disturb it, and get it tested.
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