Siding takeoffs start with net wall square footage — gross wall area minus doors, windows, and any areas not being sided — but the material-specific details after that vary a lot by product type.
Vinyl siding
Vinyl siding is sold by the square (100 sq ft) and typically needs 10% waste for a simple rectangular home, rising to 15–20% on homes with lots of corners, gables, or dormers, since each cut for an angle or trim piece adds scrap.
Fiber cement
Fiber cement (e.g., HardiePlank-style products) comes in fixed board lengths, so the takeoff should account for board-length utilization — a wall length that doesn't divide evenly into standard board lengths creates more offcut waste, and factory-primed edges at cuts often need field touch-up paint/caulk as a separate line item.
Wood siding
Wood siding (cedar, board-and-batten) is usually priced by linear footage per course rather than square footage alone, since coverage varies by reveal/overlap specified in the design — a wider reveal covers more wall per board than a narrow one, directly changing the board count.
Do not forget trim and accessories
Trim, corner boards, starter strips, and J-channel around openings are separate line items from the field siding material and are easy to under-order if the takeoff only accounts for the flat wall area.