Roofing Calculator

How roofing quantities are calculated

Roofing is sold by the square — 100 sq ft of finished roof — and shingles come bundled, almost always 3 bundles to a square:

Squares = roof area ÷ 100
Bundles = squares (incl. waste) × 3, rounded up
Underlayment rolls = roof area (incl. waste) ÷ 400 sq ft — one standard roll covers about 4 squares after overlaps

Where pitch comes in: a sloped roof has more surface than the footprint under it. The slope factor converts flat area to sloped area:

Slope factor = √(1 + (rise ÷ 12)²) — so a 6/12 pitch multiplies the footprint by about 1.118, and a steep 12/12 pitch by about 1.414.

Estimate limits, stated plainly: the dimensions mode assumes a simple roof shape over a rectangular footprint. It does not add eave or rake overhangs (add a foot or two to length and width if your eaves overhang), and complex roofs with multiple sections should be measured section by section and added up — the direct-area mode takes that total. Hips and valleys don't change the area much, but they raise cutting waste, which is why the 15% option exists. Ridge cap shingles and starter strips are separate line items — roughly one bundle of cap per 35 linear feet of ridge.

Frequently asked questions

How do I find my roof pitch? In the attic, hold a level against a rafter, mark 12 inches along it, and measure straight down from that mark to the rafter — that drop is the rise. Or use the roof pitch calculator on this site with any rise/run you can measure safely from a ladder at the gable end.

Are all shingles 3 bundles per square? Standard architectural (laminate) shingles are; some heavyweight designer shingles take 4–5 bundles per square. Check the bundle label — it states the coverage — and divide the squares above accordingly.

Can I shingle over the old roof? Many codes allow one overlay, but it voids some warranties, hides deck damage, and adds a lot of weight. This calculator gives the material for the new layer either way.