How this insulation calculator works
Insulation is sold by the package — a batt, a roll or a bag of blown-in (loose-fill) material — and each package is rated to cover a certain number of square feet. So the job is to work out your area and divide by that coverage. The principle is simple: packages needed = area ÷ coverage per package, rounded up. First find the area you are insulating. You can enter it directly, or give the length and width and let the calculator multiply them for you.
For batts and rolls, the coverage per package is fixed by the size of the blanket (a common bag covers around 40 sq ft), and you add a waste allowance for cuts and trimming, then round up. For blown-in insulation the coverage depends on the target R-value: the higher the R-value you want, the thicker the layer and the fewer square feet each bag covers — so the bag label lists a coverage figure for each R-value. Enter that figure and the calculator returns the number of bags.
Common attic R-value guidance by climate
The table below is general guidance only for attic insulation — the right R-value depends on your climate zone, your home and current code. Always confirm the required R-value with your local building code.
| Climate | Typical attic R-value |
|---|---|
| Hot / mild (southern) | R-30 to R-49 |
| Mixed / temperate | R-38 to R-60 |
| Cold (northern) | R-49 to R-60 |
Reference note: results are a planning estimate. Coverage per package and the coverage-at-R-value for blown-in insulation come from the product label and vary by product, settled thickness and installation. This estimate does not subtract framing, obstructions or openings unless you adjust the area yourself. Measure carefully and confirm coverage and the required R-value on the packaging and with your local building code before buying.
Frequently asked questions
- How much insulation do I need?
- Divide the area you are covering by the coverage of one package, then round up. For batts and rolls, packages equal the area divided by the square feet each bag or roll covers, plus a waste allowance for cuts. For blown-in, bags equal the area divided by the coverage the bag provides at your target R-value. Enter your square footage or dimensions above and the calculator returns the package count.
- What is an R-value?
- R-value measures how well insulation resists heat flow — the higher the R-value, the better it slows heat moving in or out. It depends on the material and the layer thickness. Building codes set minimum R-values for attics, walls and floors by climate zone, and product labels list the R-value you reach at a given thickness or coverage.
- What R-value do I need for an attic?
- Attics generally call for roughly R-30 to R-60 depending on climate, with colder regions at the higher end. As a rough guide, mild southern climates often target around R-30 to R-49 while cold northern climates aim for R-49 to R-60. This is general guidance only — always confirm with your local building code.
- How many bags of blown-in insulation do I need?
- Take the area you are covering and divide it by the coverage the bag provides at your chosen R-value, then round up. The bag label lists the maximum square feet one bag covers at each R-value, and that coverage drops as the target R-value rises because the material goes on thicker. Enter the bag's coverage-at-R-value figure above and the calculator returns the number of bags.
- Batts versus blown-in: which should I use?
- Batts and rolls are pre-cut blankets that fit quickly between open, evenly spaced joists and studs and are easy to handle as a DIY job. Blown-in (loose-fill) insulation is machine-blown and fills irregular spaces, gaps and existing cavities more completely, which makes it popular for attics and topping up older insulation. Many attics use batts plus blown-in on top to reach the target R-value.
- How is insulation coverage determined?
- Coverage is the square footage one package covers, printed on the product label. For batts and rolls it is fixed by the dimensions of the blanket. For blown-in it depends on the target R-value: the higher the R-value, the thicker the layer and the fewer square feet each bag covers, so the label shows a coverage figure for each R-value. Always read the label for the product you are buying.