Shingles are bought by the bundle, so this view counts bundles directly at three per square. Steep or cut-up roofs waste closer to 15% than 10% — the valleys and hips eat the extra.
Field notes
Order by the square but buy in whole bundles — three bundles cover a square of architectural shingles, and you'll want the extra for ridge caps and starter courses the footprint math doesn't include. Steep and cut-up roofs (valleys, hips, dormers) waste closer to 15% than 10%. If you can't safely measure the roof itself, measure the footprint and let the pitch factor do the work, then add a bundle.
How the shingle calculator works
The math behind the tool:
roof area = footprint area × slope factor(pitch)
squares = roof area ÷ 100 × (1 + waste%)Sources: National Roofing Contractors Association (roofing squares and slope terminology).