Attic insulation bag & depth calculator
Punch in the attic area and the R-value you are shooting for: you get the fill depth in inches and the number of bags to buy off the coverage-per-bag chart on your product.
Calculator
A 1,200 ft² attic to R-49 takes about 43 bags, roughly 19.6 inches deep. Coverage-per-bag at R is a labeled manufacturer chart — read your bag; blow a little deep to allow for settling.
Formula
fill_depth_in = target_R ÷ R_per_inch
bags = ceil( attic_area_ft² ÷ coverage_per_bag_at_R )
Two separate labeled numbers off the bag drive this: the R per inch tells you how deep to blow, and the coverage-per-bag at that R tells you how far one bag goes. Higher R = fewer square feet per bag, because each bag has to go on deeper. The ceiling rounds up — you buy whole bags.
Worked example
A 1,200 ft² attic to R-49 with blown fiberglass at R-2.5/in, and a bag rated 28 ft² at R-49:
Depth: 49 ÷ 2.5 = 19.6 in. Bags: ceil(1,200 ÷ 28) = ceil(42.9) = 43 bags.
So order 43 bags and set the depth ruler at roughly 20 inches. Blow a touch deep — loose fill settles, and the coverage chart assumes the installed (settled) thickness.
Reading the bag & blowing it even
Use the chart on the bag you actually buy. Coverage-per-bag at a given R is printed on every bag of loose fill and it varies by product — blown fiberglass and cellulose have different charts. The default here (28 ft²/bag at R-49) is a labeled fiberglass typical; cellulose is denser and its chart reads differently, so read yours.
Depth vs. bags are a cross-check, not a coincidence. If the bag count and the fill depth disagree with what the machine is laying down, something is off — usually the blower is over- or under-fluffing. Mark the depth on rafter-mounted rulers around the attic before you start so you can hit the target evenly.
Settling and coverage. Loose-fill fiberglass barely settles; cellulose settles more, which is why its charts already bake in a settled thickness. Either way, aim slightly high rather than exactly on the line, and keep it even — a thin spot at R-30 in an otherwise R-49 attic drags the whole ceiling down.
Before you blow: install eave baffles so the fill does not choke the soffit vents, box out recessed lights not rated IC, dam the attic hatch, and mark the joists you need to keep clear. This tool sizes the material; it does not replace ventilation and fire-clearance detailing, which follow the data sheet and local code.
Reference table
Labeled blown-fiberglass coverage per bag at each target R (read your own bag — charts vary by product).
| Target R-value | Coverage per bag |
|---|---|
| R-13 | 108 ft²/bag |
| R-19 | 74 ft²/bag |
| R-30 | 47 ft²/bag |
| R-38 | 37 ft²/bag |
| R-49 | 28 ft²/bag |
| R-60 | 23 ft²/bag |
Higher R = deeper fill = fewer ft² per bag. Labeled planning typicals.