Squares and bundles of shingles based on roof area and pitch.
One roofing square = 100 square feet of roof surface. A typical 1,500 sq ft house with a 6/12 pitch has approximately 1,800 sq ft of roof surface (18 squares). Architectural shingles typically come 3 bundles per square, each bundle covering ~33 sq ft.
Always include 15% waste for valleys, hips, and cut waste. Roofing is a high-risk DIY project — always use proper fall protection. Most municipalities require a permit for full roof replacements.