A square is 100 square feet
Roofing is priced and ordered in squares: one square covers 100 square feet of roof surface. Note surface — slope-adjusted area, not the building footprint. A 2,000 sq ft footprint at 6:12 pitch is roughly 2,240 sq ft of roof, or about 22.4 squares before waste.
Bundles are packaging, not measurement
Architectural shingles are typically packaged three bundles to a square. So 22 squares at 10% waste is 24.2 squares — 73 bundles after rounding up. The bundle count is arithmetic; the waste percentage is judgment. Cut-up roofs with valleys and hips waste more than simple gables.
The accessories are the estimate
Starter strip follows eaves and rakes. Drip edge follows the same edges. Ice and water membrane goes where code and exposure demand. Ridge vent follows the ridges. Boots and flashing kits count the penetrations. None of these come from area — they come from measured edges, and they are the lines most often missing from quick estimates.
Drevanto's certified asphalt-shingle-roof assembly works exactly this way: measured plane area plus measured edges and penetrations produce the material lines. The certified example shows all ten canonical materials for a 2,000 sq ft replacement.