Squares, bundles, and the conversion
Roofing is sold and measured in squares. One square covers 100 square feet, and for standard three-tab and architectural shingles it takes three bundles to make a square.
Enter the roof area measured on the roof plane, not the building footprint. A footprint measurement ignores pitch entirely and will under-order every time, and the steeper the roof the worse the error.
Waste rises with complexity
A simple gable runs about 10% waste. Hip roofs land near 12% because every hip requires cutting. Complex roofs with multiple valleys reach 15%, and heavily cut-up roofs with many penetrations justify 18%.
The waste is cutting loss, and cutting loss scales with how many times the plane changes direction. Pick the complexity that honestly matches the roof rather than the one that makes the number look better.
What this does not include
Field shingles only. You still need starter strip at roughly two bundles per 100 linear feet of eave, ridge cap at about one bundle per 35 linear feet of hip and ridge for cap shingles, drip edge, ice and water shield, underlayment, and fasteners.
Order one square beyond the calculation and set it aside. Shingle lots vary in color, and a matching repair three years out is worth far more than the cost of the extra bundle.