Measure roof planes, not only the building footprint. Pitch, hips, valleys, dormers, penetrations, cut complexity, and package rounding all affect the order.
Calculate each plane
Measure or derive the sloped area of every plane, then add them before applying waste. Keep the geometry visible so the takeoff can be checked.
Order by package and coverage
Convert total squares using the manufacturer's coverage per bundle or roll and round to whole packages. Hip and ridge products may have different linear coverage.
Price the whole roof system
Include tear-off, disposal, deck repairs, underlayment, ice barrier, flashing, penetrations, ventilation, delivery, lift or staging, cleanup, overhead, and profit.