Insulation Calculator
Work out how many fiberglass batts or bags of blown-in cellulose you need to hit a target R-value over a given area — pick the R-13 to R-60 level for your walls, ceiling, or attic and get the package count to buy.
How to Use
- Enter the area to insulate in square feet — the wall, floor, or ceiling surface.
- Choose the target R-value for that assembly, from R-13 for a 2×4 wall up to R-49 or R-60 for an attic.
- Pick the material type: fiberglass batts or blown-in cellulose.
- Read how many batt packs or bags the job needs based on coverage per package.
- Check the Show Work below and add a little extra for trimming and gap-filling.
Show Work
Coverage per bag/roll
About the Insulation Calculator
Need a hand with building, materials estimation and site work? The Insulation Calculator does the work for you — free, and right here in your browser. Work out how many fiberglass batts or bags of blown-in cellulose you need to hit a target R-value over a given area — pick the R-13 to R-60 level for your walls, ceiling, or attic and get the package count to buy.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What R-value do I need?
It depends on the assembly and your climate. A 2×4 wall typically takes R-13, a 2×6 wall R-19 to R-21, and attics run R-38 to R-60 depending on how cold your region gets. Check your local code or the DOE climate-zone recommendations, then pick that R-value here to size the material.
What is the difference between batts and blown-in?
Batts are pre-cut fiberglass blankets sized to fit between studs and joists — best for open walls and accessible floors. Blown-in cellulose or fiberglass is loose fill machined into attics and cavities, which fills gaps better and is ideal for adding depth on an attic floor. This tool sizes both by coverage per package.
Can I just stack more insulation to raise R-value?
For attics, yes — adding depth of blown-in or laying unfaced batts over existing insulation raises the total R-value, and you can layer to hit R-49 or R-60. In walls you are limited by cavity depth, so a 2×4 wall caps out near R-15 with standard fiberglass unless you switch to a denser product.
Why is my actual performance lower than the R-value?
Compressed batts, gaps around wiring and boxes, and missed spots all cut real-world performance — insulation only works when it fully fills the cavity without voids. Buy a little extra so you are not tempted to stretch or compress batts, and seal air leaks first, because air movement undermines any R-value.
Should faced or unfaced batts be used?
Faced batts have a kraft-paper or foil vapor retarder that goes toward the heated living space, used in walls and floors over crawl spaces. Unfaced batts are for adding a second layer over existing insulation or where a separate vapor barrier exists. Putting two vapor retarders in one assembly can trap moisture, so use unfaced for top-up layers.
How do I use the Insulation Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Topping up attic insulation
Size the bags of blown-in needed to bring an attic up to R-49 or R-60 for lower heating bills.
Insulating new framed walls
Count R-13 or R-19 batt packs for 2×4 or 2×6 walls in a remodel or addition.
Finishing a basement or floor
Estimate batts for a floor or ceiling assembly at the R-value your code requires.
Budgeting an energy upgrade
Translate area and target R-value into package count so you can price the materials.
Comparing batts vs blown-in cost
Run the same area both ways to see which material is cheaper to reach your R-value.
Last updated: