Roofing Calculator
Squares, bundles, underlayment, and ridge cap — from the footprint and the pitch, the two numbers you can get from the ground.
The multiplier turns flat footprint into sloped roof surface. Measure pitch with a level and a tape on the roof, or read it off the plans — do not guess it from the ground.
Planning math only: 3 bundles to a square and ~35 lineal feet of ridge cap per bundle are the standard shingle conventions. Valleys, hips, starters, and drip edge are not in this count — walk the roof before you order.
How the math works
A roof’s flat footprint is not its surface. Pitch stretches it: roof area = footprint × the pitch multiplier (a 6/12 roof multiplies by 1.118; a 12/12 doubles the slope factor to 1.414). A “square” is 100 sq ft of roof. Three bundles of shingles make a square. Underlayment rolls cover roughly 1,000 sq ft (synthetic), 400 sq ft (#15 felt), or 200 sq ft (#30 felt) once laps are counted. Ridge cap runs about 35 lineal feet per bundle.
A worked example
A 40 ft × 30 ft house at 6/12 pitch: 1,200 sq ft of footprint × 1.118 = 1,342 sq ft of roof = 13.42 squares. With 10% waste: 45 bundles of shingles and 2 rolls of synthetic underlayment. A 40 ft ridge takes 2 bundles of ridge cap. Enter it above — the calculator agrees.
Questions from the field
How many bundles are in a square? Three, for standard architectural shingles. A 13.42-square roof at 10% waste needs 45 bundles.
How do I convert roof pitch to area? Multiply the flat footprint by the pitch factor: 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414.
How much waste should I add for shingles? 10% for a simple gable roof, 15% for hips, valleys, and cut-up roofs. The waste is in the cutting, not the covering.
Does this count starter strips, valleys, and flashing? No — this is the field-and-cap count. Valleys, starters, ice-and-water shield, and metals get measured on the roof; the calculator says so on its own output, because a count that quietly omits them is worse than no count.
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