Insulation Calculator
Count bags of blown-in cellulose or fiberglass, or packages of batts, for your area and target R-value — plus the depth to blow to.
How much insulation do I need?
Insulation is bought by coverage: each bag of blown-in material covers a stated area at a stated R-value — blow it deeper for more R and the same bag covers less floor. So the count is the area (with an allowance for settling and uneven blowing) divided by the coverage per bag at your target R.
Bags = area (with waste) ÷ coverage per bag at your R-value
Example: a 500 sq ft attic taken to R-30 with blown cellulose: at R-30 a bag covers about 11.9 sq ft, so 550 sq ft (with 10% in) needs 47 bags, blown about 8.4 inches deep.
Coverage per bag, by material and R-value
| Target | Cellulose (sq ft/bag) | Blown fiberglass (sq ft/bag) | Depth — cellulose | Depth — fiberglass |
|---|
| R-13 | ≈ 27.8 | ≈ 77.5 | ≈ 3.6 in | ≈ 4.9 in |
| R-19 | ≈ 18.9 | ≈ 52.6 | ≈ 5.3 in | ≈ 7.2 in |
| R-30 | ≈ 11.9 | ≈ 32.4 | ≈ 8.4 in | ≈ 11.4 in |
| R-38 | ≈ 9.4 | ≈ 25.1 | ≈ 10.6 in | ≈ 14.4 in |
| R-49 | ≈ 7.4 | ≈ 19.4 | ≈ 13.7 in | ≈ 18.6 in |
These are typical manufacturer chart figures — the chart printed on your bag overrules them, so check it before a big order; bag sizes differ between brands. Fiberglass covers more per bag but needs more depth for the same R; cellulose is heavier and settles, which is one reason the allowance here is worth keeping. For batts and rolls, packages are counted at 40 sq ft of coverage each — again, read the package: batt bags vary widely by width and R.
Attic rulers are the cheapest accuracy tool in the job: staple a few depth markers to the trusses before blowing, so the depth at the far corner is the depth you paid for.
Frequently asked questions
What R-value should my attic have?
Most US guidance lands at R-38 to R-49 for attics in cold climates (zones 5–8), R-30 to R-38 in moderate ones, and R-30 in the hot south — with floors at R-19 to R-30 and walls at R-13 to R-21. Your local energy code states the required minimum for new work.
Can I blow new insulation over the old?
Usually yes, and it is the normal top-up job — the R-values add. Flatten badly matted or water-damaged insulation first, and never bury knob-and-tube wiring or unsealed recessed lights without checking their ratings; some fixtures must stay uncovered or be boxed off.
Cellulose or fiberglass — which is better?
Both reach the same R at their chart depths. Cellulose is denser, settles a little, and damps sound better; fiberglass is lighter (a real consideration for weak ceilings) and doesn't settle. Cost per R in your area usually decides it.
Why does the count include a waste allowance?
Blown insulation settles after installation (cellulose by up to ~10–20% over time at its loose density charts, which bag charts already anticipate in part) and corners blow thin. The 10% here keeps the far corners honest. You can lower it for batts, where coverage is exact.
CalcCoverage