Insulation Calculator
Calculate how many batts or rolls of insulation to buy.
Check the coverage printed on the package \u2014 a typical R-13 roll covers about 40 sq ft. 5% waste included.
Insulation is one of the few home projects that literally pays you back — but only the parts you actually cover. Running out two joist bays from the wall on a Sunday evening is how attics end up with cold spots for years. Our insulation calculator above turns your area and your product’s package coverage into an exact package count, with 5% waste included.
The Calculation
Packages = (Area to insulate × 1.05) ÷ Coverage per package, rounded up
The formula is simple because manufacturers do the hard part for you: every bundle of batts and every roll states its coverage in square feet on the label. A typical R-13 kraft-faced roll covers about 40 sq ft; an R-30 batt bundle often covers 50–60 sq ft. Put the real number from your chosen product into the calculator — coverage varies widely between R-values and brands, because higher R-value means thicker material and less of it per package.
Measuring Your Area
Attic floors: length × width of the attic footprint. Don’t subtract for framing — the overlap is your waste margin.
Walls: wall length × height per wall, minus large openings like windows and doors. For a whole exterior, perimeter × wall height minus openings gets you close.
Floors and crawl spaces: the same footprint math as attics.
For oddly shaped spaces, split the area into rectangles and add them up. Round up on every measurement — insulation compresses into slightly small spaces far better than it stretches into gaps.
Choosing an R-Value
R-value measures resistance to heat flow — higher is warmer. Recommendations vary by climate zone, but common US targets are:
- Attics: R-38 to R-60 (cold climates need the top of that range)
- Exterior walls (2×4): R-13 to R-15
- Exterior walls (2×6): R-19 to R-21
- Floors over unheated spaces: R-19 to R-30
If your attic already has some insulation, you can add unfaced batts or rolls on top, laid perpendicular to the joists. Never stack a vapor-barrier (kraft/foil) face in the middle of an insulation sandwich — trapped moisture is how mold starts. Facing belongs against the warm-in-winter side, and only in the first layer.
Batt Width Matters
Batts come sized to standard framing: 15-inch widths for 16-inch on-center framing, 23-inch widths for 24-inch on-center. Measure your joist or stud spacing before buying — the wrong width either leaves gaps (kills performance) or requires cutting every batt lengthwise (kills your weekend). Insulation performs at its rated R-value only when it fills the cavity snugly without being compressed.
Installation Tips
- Wear protection: long sleeves, gloves, goggles, and a dust mask. Fiberglass itch is real and lasts days.
- Cut with a utility knife against a board, compressing the batt with a straightedge.
- Don’t block soffit vents in attics — use baffles to keep the airflow path open.
- Split batts around wires and pipes rather than stuffing them behind; compression creates cold spots.
- Air-seal first: caulk and foam the gaps around penetrations before insulating. Insulation slows heat; it doesn’t stop drafts.
Frequently Asked Questions
How many rolls for a 500 sq ft attic? With 40 sq ft rolls: 500 × 1.05 ÷ 40 = 14 packages. With 60 sq ft R-30 bundles: 9 packages.
Is it OK to compress insulation to fit? No — R-value drops when compressed. An R-19 batt squeezed into a 2×4 wall performs around R-13 anyway; buy the batt made for the cavity depth.
Faced or unfaced? Faced for first layers against living space in most climates; unfaced for top-up layers and sound walls. Local practice varies with climate — when in doubt, ask your building department.
Estimates are for planning purposes. Check coverage on your specific product’s label, and consult local code for required R-values.
Notes from building this calculator
Batt insulation is sold by the bag in square feet of coverage, but the coverage printed on the bag is gross, not net. Framing occupies roughly 10 to 15 percent of a stud wall face at 16-inch centers, and batts do not go there. We estimate against cavity area rather than gross wall area, then round up by the bag, since the bag is the only unit you can actually buy.
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Sources and Further Reading
Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. Assembly and dimensional references come from applicable model building codes. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 7, 2026