Pitch is rise over a 12-inch run — this view turns your rise/run into an angle and the slope factor that scales roof area above the flat footprint. A 6/12 roof carries about 12% more surface than its footprint.
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 roof pitch 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).