Measure the house footprint (or each roof plane), pick the pitch and see the real roof area, squares and how many bundles to order.
Roof area = footprint × pitch factor, where the pitch factor is √(1 + (rise ÷ 12)²): 1.118 for a 6/12 roof. One square is 100 sq ft. Most architectural shingles come 3 bundles per square. Add 10–15% waste for hips, valleys and starter.
For complex roofs, measure each plane separately and add them up.
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See the Roofing Estimator