By Zencalcy Editorial TeamLast reviewed About us
Roofers price in squares of 100 square feet, and a roof is always larger than the floor beneath it because of the pitch. The slope multiplier converts the footprint to true roof area; then waste, valleys and hips push the order up another 10–15%.
Roof area, squares and material cost from the footprint.
Roof area = footprint × (1 ÷ cos(atan(pitch ÷ 12))) Squares = area ÷ 100
Footprint should include the eaves overhang, typically 12–24 inches on each side. Three bundles make a square for standard three-tab shingles; architectural shingles usually need three as well, but check the bundle coverage.
| Symbol | Meaning |
|---|---|
| A | Building footprint area. |
| m | Slope multiplier. |
| S | Squares of roofing. |
1,200 sq ft × 1.118 = 1,342 sq ft, or 13.4 squares; with 10% waste, 15 squares and 45 bundles.
Continue with the Roof Pitch Calculator, Rafter Length Calculator, Roof Truss Calculator, or the Roof Shingle Calculator — or browse more roofing and all construction calculators.
Three for most asphalt shingles. Always confirm on the bundle label; some products are four bundles per square.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026