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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Enter the area, the roll size you are buying and the cavity height you will cut to. You get the roll count two ways — the area answer for an attic run and the cut-piece answer for walls — plus the offcut each roll leaves behind.
With these inputs:
| Result | Value |
|---|---|
| Rolls by area — attic run | 22 rolls |
| Rolls cut to height — walls | 23 rolls |
| Whole pieces per roll | 4 pieces |
| Offcut left on each roll | 12 in |
| Coverage to buy | 880 sq ft |
| Estimated cost | $874 |
Coverage per roll = (roll width ÷ 12) × roll length · Rolls by area = area × (1 + waste %) ÷ coverage per roll, rounded up · Whole pieces per roll = (roll length × 12) ÷ cavity height, rounded down · Offcut per roll = (roll length × 12) − (whole pieces × cavity height) · Rolls cut to height = pieces needed ÷ whole pieces per roll, rounded up
| Roll length | 15 in wide | 23 in wide |
|---|---|---|
| 16 ft | 20 sq ft | 30.7 sq ft |
| 20 ft | 25 sq ft | 38.3 sq ft |
| 25 ft | 31.3 sq ft | 47.9 sq ft |
| 32 ft | 40 sq ft | 61.3 sq ft |
| 39.2 ft | 49 sq ft | 75.1 sq ft |
| 40 ft | 50 sq ft | 76.7 sq ft |
Width times length, nothing more. The exact length of the roll you are buying is printed on the packaging — enter it above rather than assuming.
| Cavity height | Whole pieces per roll | Offcut left | Wasted |
|---|---|---|---|
| 48 in | 8 pieces | 0 in | 0% |
| 93 in | 4 pieces | 12 in | 3.1% |
| 96 in | 4 pieces | 0 in | 0% |
| 105 in | 3 pieces | 69 in | 18% |
| 120 in | 3 pieces | 24 in | 6.3% |
| 144 in | 2 pieces | 96 in | 25% |
Offcuts shorter than a full cavity can still be stuffed into cripple bays above doors and below windows, so treat this as the worst case, not a guarantee.
Multiply the width in inches by 12, then by the length in feet. A 15 in wide roll 32 ft long covers 40 sq ft; the same length in 23 in covers 61.3 sq ft. Both numbers are printed on the packaging, and higher R-values come in shorter rolls because the material is thicker on the spool.
At 40 sq ft per roll and 10% waste, 1,000 sq ft needs 1,100 sq ft of coverage, which is 28 rolls. Wider 23 in rolls covering 61.3 sq ft each bring that down to 18. An attic is the easy case because you roll out along the joists and cut once at the end of each run.
Same material, different packaging. Batts arrive precut to a fixed length — usually 93 in or 48 in — so you count pieces. Rolls arrive as one continuous length you cut yourself. Rolls are normally cheaper per square foot and better for long uninterrupted attic runs; batts are faster in walls because the cutting is already done.
Because a roll rarely divides evenly into cavity heights. A 32 ft roll is 384 in. Cut into 93 in wall lengths it gives four pieces and leaves 12 in over. Cut into 105 in lengths for a 9 ft wall it gives three and leaves 69 in over — 18% of the roll gone. Area alone cannot see that, which is why this calculator shows both.
Often, yes. Cripple bays above doors and below windows are short, and so are the triangular bays in a gable end. Keep offcuts until the job is finished rather than binning them as you go — most walls have enough short bays to absorb them.
15 in. The nominal 16 in spacing measures centre to centre, so the clear gap between studs is about 14.5 in; a 15 in roll friction-fits into it. For 24 in on-center framing you want 23 in. Buying the wrong width means trimming every single piece lengthways.
Ten percent is the usual allowance and it is what this calculator defaults to. Push it to 15% if the space has a lot of obstructions — wiring, pipes, bracing, odd-width bays — because each of those means a cut and every cut leaves a piece you may not be able to place.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.