Estimate shingle bundles from your roof area in squares, with a waste factor for cuts and starter courses.
bundles = squares × bundles_per_square × (1 + waste)
Three bundles of standard 3-tab shingles cover one square (100 ft²). Use the roofing calculator to get your square count from the footprint and pitch.
Add extra bundles for starter strips and ridge caps, which the waste factor partly covers.
If the roofing calculator told you the roof is 25 squares, standard 3-tab shingles at 3 bundles per square need 75 bundles before waste. With 10% waste that's about 83 bundles.
Architectural (dimensional) shingles sometimes run a bit heavier per square — check the wrapper, as some need 4 bundles per square. Also budget separately for hip and ridge caps and starter strip, which are sold as specialty bundles.
Three bundles per square is standard for 3-tab; architectural shingles vary by product. Get squares from the roofing calculator.
Three for standard 3-tab shingles; some architectural shingles need four.