Common residential pitches: 4/12 (low slope), 6/12 (standard), 8/12 (steep), 12/12 (45°, very steep). Pitches under 2/12 are considered low-slope or flat and need different roofing systems (TPO, EPDM) than shingles.
The pitch multiplier converts plan area (the building's footprint when viewed from above) to actual roof surface area. The multiplier equals sqrt(1 + (rise/12)²). For 6/12, multiplier ≈ 1.118; for 12/12 ≈ 1.414.
On the same roof you may have multiple planes with different pitches — a main gable at 6/12, dormers at 8/12, a porch at 3/12. Each plane's surface area uses its own multiplier.