Underlayment Coverage Calculator
Underlayment, whether roofing felt, synthetic membrane, or a floor foam or moisture barrier, is sold by the roll with a printed coverage in square feet. To size a job you divide the area to cover by the usable roll coverage, add an allowance for seam overlaps and trimming, and round up to whole rolls. This calculator does that with your own roll coverage and overlap figures, so the count matches the exact product you are buying. It reports both the exact rolls and the rounded count to buy.
Underlayment formula
Coverage needed = area * (1 + overlap% / 100)
Rolls (exact) = coverage needed / roll coverage
Rolls to buy = ceil(rolls exact)
Spare coverage = rolls to buy * roll coverage - coverage needed
The overlap allowance accounts for lapped seams and trimming, which reduce usable coverage below the printed roll figure. Spare coverage shows how much extra the rounding leaves you for repairs.
Underlayment context
- Roll coverage is printed on the product, usually roll width times length in square feet.
- Seams and ends overlap to shed water, which consumes material and lowers usable coverage.
- Roofing felt often laps several inches per course; floor underlayment laps less.
- The same arithmetic covers roof felt, synthetic membrane, and floor foam or moisture barriers.
- Round up to whole rolls and keep the spare for repairs and future patches.
Underlayment: frequently asked questions
How many rolls of underlayment do I need?
Divide the area to cover by the usable coverage of each roll, add a waste or overlap allowance, and round up. Roll coverage is printed on the product (for example 100 or 200 square feet per roll). Seams overlap, which reduces usable coverage, so include an overlap allowance for an accurate count.
What is roll coverage?
Roll coverage is the area one roll covers, usually stated in square feet on the packaging (often the roll width times its length). It is the gross area before you account for overlaps at seams and trimming at edges, so the usable coverage on the job is slightly less than the printed figure.
Why include overlap?
Underlayment seams are lapped so water cannot pass between courses, and ends are lapped too. Each lap consumes material, reducing usable coverage. A typical lap allowance is a few percent for floor underlayment and more for roofing felt, where laps can be several inches per course.
Does this work for roof felt and floor underlayment?
Yes. The arithmetic is the same: area to cover divided by usable roll coverage, with an overlap or waste allowance. Enter the coverage from your specific product, whether it is roofing felt or synthetic, floor foam, cork, or a moisture barrier, so the rolls match what you are buying.
Should I round up to whole rolls?
Yes. You cannot buy a partial roll, and running short mid-job risks a delay and a possible batch mismatch. This calculator reports the exact rolls and the rounded-up count to buy, so you can see how much extra the rounding adds and decide whether to keep a spare roll.
Official sources
- International Code Council: International Residential Code (underlayment requirements).
- U.S. National Institute of Standards and Technology: NIST Unit Conversion (area units).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.