Start with finished geometry
Use the outside length, width and thickness of each finished rectangular part. If edging changes the outside dimensions, use the dimensions after that edging rather than the smaller saw blank. Keep a reference label that can be found on the physical part. In this planner, length runs horizontally on the map and width runs vertically; top and bottom are length edges, while left and right are width edges. Front and back refer to the two broad faces. This convention is a drawing convention, not a promise about which way a door will be installed.
Count only the selected surfaces
One broad face has area length × width. Each top or bottom edge has area length × thickness. Each left or right edge has area width × thickness. Select the surfaces that the approved finish schedule actually coats, then multiply their sum by the number of identical parts. The planner shows face area and edge area separately so an unexpected edge total can be checked. It does not assume that a hidden face is bare, or that a visible face necessarily needs the same product as an edge. Make that decision from the project specification.
Work through a shelf example
A 600 × 300 × 18 mm shelf has a broad-face area of 0.18 m². Two faces are 0.36 m². One selected length edge adds 600 × 18 / 1,000,000 = 0.0108 m². The selected total is therefore 0.3708 m² per shelf. Four identical shelves produce 1.4832 m² for one coat. A second coat changes the coated-area demand to 2.9664 m²; it does not change the physical surface area. This difference is why the result displays both selected area and area multiplied by coats.
Split rows when treatments differ
If a door front receives stain and topcoat but its back receives topcoat alone, enter two rows using the same physical dimensions. On the first row select the front face and products A and B. On the second select the back face and product A only. Put the relevant edge treatment on one of those rows or on a third edge-only row. Label the rows “Door — front”, “Door — back” and “Door — edges” so a reviewer can see that they describe disjoint surfaces of the same part. Never select the same surface twice for the same product unless it represents an intentionally separate coating stage.
Keep the geometry boundary visible
This model covers flat rectangular faces and straight rectangular edges. It does not deduct holes, add the inside of cutouts, follow routed profiles, or model turned legs and curved surfaces. A large opening may make the rectangle an overestimate of face area while its reveal adds area that is absent from the model. Measure such work separately rather than forcing it into a clean panel rectangle. Before exporting, compare the surface map against the assembly drawing and check quantity, thickness, labels and product assignment.