Roofing Calculator

Multiply the building footprint (length × width) by the slope factor for your pitch. The slope factor is √(rise² + 12²) ÷ 12. A 4/12 pitch has a factor of about 1.054, so a 1,200 sq ft footprint has roughly 1,265 sq ft of roof surface.

A square is 100 square feet of roof area — the standard unit roofers use to price and order material. A 2,000 sq ft roof is 20 squares. Shingles are usually sold three bundles to the square.

Most architectural and 3-tab shingles come three bundles per square (100 sq ft). Divide your roof area by 100 to get squares, then multiply by 3, rounding up. Always add 10–15% for waste, hip and ridge caps, and starter courses.

Pitch is the rise in inches for every 12 inches of horizontal run. A 6/12 pitch rises 6 inches per foot (about 26.6°). Steeper pitches have a larger slope factor, so they need more material than the footprint alone suggests.

Add about 10% for a simple gable roof and 15% or more for a complex roof with many hips, valleys, and dormers. The waste field lets you include this. It is cheaper to have a little extra than to run short mid-job.