Insulation Calculator
Calculate how many packages of fiberglass batt insulation you need for a given R-value.
How we calculate it
Required area = Area × (1 + Waste factor), Packages = ceil(Required area / Coverage per package), where coverage per package depends on the R-value chosen.
Understanding your estimate
A stack of fiberglass batts in the garage does nothing for a wall's R-value if the package count doesn't match the space you're actually filling. Come up short and the last few stud bays sit open until a second trip to the store, with cold air moving through those gaps in the meantime. Buy too many and the extra rolls just sit there, because batt insulation doesn't go back on the shelf once it's out of the wrapper. This calculator starts from the R-value you've already chosen for the job, not a thickness a formula happens to invent, and it only ever rounds the package count in one direction.
A 500 ft² job, worked out
Say you're insulating 500 ft² of wall cavity with R-13 batts, the default this calculator opens with, and the default 10% waste factor.
- Add the waste factor first: 500 × 1.10 = 550 ft². That's the area you shop for, not the raw square footage of the wall.
- R-13 batts cover 106.56 ft² per package, one Owens Corning SKU, unfaced. 550 ÷ 106.56 = 5.16, rounded up to 6 packages.
- Switch the dropdown to R-19 or R-21 without touching anything else and the same 550 ft² needs 550 ÷ 77.5 = 7.09, rounded up to 8 packages. R-30 lands at 550 ÷ 80 = 6.875, rounded up to 7.
- Six packages, eight, or seven, same wall, same waste factor. The R-value you pick changes what you buy just as much as the square footage does.
Why a dropdown of R-values, not a generic thickness
Some formulas start from an R-per-inch constant and work backward into a thickness, then leave you to find a product that happens to measure that thick. This calculator goes the other way. Pick R-13, R-19, R-21, or R-30 first, the four values with package coverage confirmed against a real manufacturer SKU, and the coverage per package for that exact rating is already built in. That match matters because batt insulation ships in fixed R-values with fixed dimensions. A thickness a formula invents doesn't help at the store if nothing on the shelf is cut to it.
Why R-19 and R-21 need the same package count
Pick R-19 for a standard 2x6 wall or R-21 for the high-density version of that same cavity, and the package count for a given area comes out identical: 77.5 ft² per package either way. That's not a rounding coincidence. Both ship in runs of ten bags covering 775 ft² total from the same manufacturer line, so the coverage per bag matches to the decimal. The R-value tells you how the wall performs once it's insulated. The package count tells you how much to buy. R-19 and R-21 answer the second question identically while still answering the first one differently.
Why the waste factor still defaults to 10%
Batt insulation is friction-fit. It doesn't get cut into odd shapes the way shingles or tile do, so a lower default waste factor here, or no field at all, would seem reasonable. It isn't how this calculator works. Every stud bay has an electrical box, a run of wiring, or a corner that needs a batt trimmed to fit around it, and that adds up across a whole wall even though the material itself wastes very little in the process. Ten percent is the same default this calculator's siblings use for concrete, gravel, mulch, and tile, and nothing about how batt actually gets installed argued for breaking from it. The field stays editable if a job runs into more cutting than that, a wall dense with outlets and switches, for instance.
What this calculator doesn't cover yet
Four R-values live here: R-13, R-19, R-21, and R-30, the ones with package coverage confirmed against an actual SKU. R-11, R-15, R-38, and R-49 sit on the same store shelves and are left out for now rather than filled in with a guessed coverage number. Spray foam is a different installation method with different math entirely, and it gets its own calculator. Mineral wool and rock wool batts are real products this one doesn't account for either. The coverage numbers here are fiberglass batt, specifically, not insulation in general.