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A shingle calculator turns your roof area into the number of shingle bundles to buy — field shingles plus the easy-to-forget extras: the starter course along the eaves and rakes, and the hip-and-ridge cap bundles along the peaks.
Knowing your squares isn't enough at the store: starter and ridge cap are sold separately and run out fast. Enter your ridge and eave lengths and this counts those bundles too.
Field bundles = squares × 3 × waste multiplier (3 bundles per 100 sq ft square). Hip/ridge cap ≈ one bundle per 25 linear feet; starter course ≈ one bundle per 105 linear feet of eaves and rakes. Cap and starter are extra to the field count.
A 2,000 sq ft roof is 20 squares — about 66 field bundles with 10% waste, before starter and ridge cap.