Insulation Calculator
Estimate how many bags or batts of insulation you need.
Square feet covered per bag or bundle.
Results update as you type.
How it works
An insulation calculator works out how many bags or bundles of insulation you need to cover a wall, ceiling or attic, by dividing the area to be insulated by the coverage each bag provides. Coverage is printed on every package and depends on the product's R-value and thickness: a thicker, higher-R batt fills more of each cavity and so covers fewer square feet per bag, while a thinner product stretches further. Because you can only buy whole bags, the tool rounds up. Insulation is worth estimating carefully because it directly controls a building's comfort and energy bills — gaps, thin spots and compressed batts all leak heat and undercut the rated R-value, so it pays to cover the whole area properly rather than skimp. At the same time, buying far too much wastes money and clutters the site. Entering the coverage figure from the specific product you intend to use, rather than a generic number, is what makes the estimate reliable, since coverage varies widely between materials and R-values. This calculator handles the area-based bag count; it does not adjust for framing that displaces some cavity space or for the extra material tucked around obstructions, so adding a small margin is wise. Homeowners, remodelers and insulation installers use it to plan an attic top-up or a wall job and buy close to the right amount the first time.
Bags = ⌈area ÷ coverage per bag⌉
The area you need to insulate is divided by the number of square feet a single bag or bundle covers, which is printed on the packaging and varies with the insulation's R-value and thickness. The result is how many bags of material the surface requires. Because insulation is sold only in whole bags and a partial bag still counts as a full purchase, the figure is rounded up. Higher R-value and thicker batts cover fewer square feet per bag, so entering the coverage from the exact product you plan to buy keeps the estimate accurate for your job.
Source:Standard construction estimation (area method)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter area to insulate.
- 2
Adjust the options
Adjust coverage per bag if they apply — sensible defaults are already set.
- 3
Read your result
Bags needed updates instantly as you type — no button to press.
Frequently asked questions
How do I find the coverage per bag?
The coverage is printed on the insulation packaging, usually stated as square feet at a given R-value and installed thickness. It varies widely: a high-R attic batt covers far fewer square feet per bag than a thin wall batt. Read the figure for the exact product you plan to buy and enter it here, rather than relying on a generic number, because using the wrong coverage throws the whole bag count off.
Does R-value change how much I need?
Yes, indirectly. A higher R-value means a thicker or denser product, so each bag covers fewer square feet, and you will need more bags for the same area. The R-value itself is chosen to meet the thermal performance your climate and building code require. Pick the R-value first, then read that product's coverage and enter it, so the calculator counts bags for the material you are actually installing.
Should I buy extra insulation?
A small margin is sensible. Framing members displace some cavity space, obstructions like wiring and pipes need material tucked around them, and attics have irregular edges and access points that are easy to underestimate. Adding roughly 5 to 10 percent, or simply rounding your area up, avoids a shortfall that leaves a cold gap. Insulation stores easily, so a leftover bag or two is more useful than an uninsulated patch.
Why does compressing insulation reduce its value?
Insulation works by trapping still air in its fibers, and that trapped air is what resists heat flow. Compressing a batt into a space thinner than its rating squeezes out the air pockets, lowering the effective R-value even though the material is present. That is why you should fill cavities to the intended thickness and avoid over-stuffing. Cover the full area properly rather than cramming too much material into too little space.
Does this calculator work for blown-in insulation?
It works if the product states coverage per bag at your target depth, which blown-in cellulose and fiberglass packaging does — usually as a coverage chart tied to the finished depth and R-value. Read the square feet per bag for the depth you want, enter it as the coverage, and the tool returns the bag count. For loose fill, remember that settling over time means installing slightly deeper than the final target depth.