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Start with the usable rectangle
A slab is rarely a perfect rectangle. An uneven edge, an open corner or a natural fissure can reduce the area available to a cutting plan. The outer measurement and the usable measurement answer different questions.
Before comparing quotations, ask each yard to identify the basis of measurement: gross slab area, a net rectangle, or the finished pieces on your cutting list. Put that basis in the purchase order.
Ask for a slab-by-slab sheet
Request the slab number, length, width, thickness and accepted area for every slab. Ask the supplier to show the measuring points in a continuous video with the slab ID in frame.
A simple illustration: a slab measuring 2.75 × 1.50 m has a gross area of 4.125 m². A usable rectangle of 2.65 × 1.40 m gives 3.71 m². These are example dimensions, not a standard allowance. The actual loss depends on the slab and cutting plan.
Three questions before payment
Is the quotation based on gross or net area? How are damaged corners and open defects treated? Can the final measurement sheet be approved before packing?
For a project with repeated pieces, give the fabricator your cutting list before reserving a lot. The largest slab is not always the one that yields the most usable work.