Field guide / 02—08
Calculate an L-shaped room without double counting
Split the plan into non-overlapping rectangles and add their areas.
Sketch the split
Draw a line that divides the L shape into two rectangles with no overlap. Label them A and B. Record which lengths were measured and which were obtained by subtraction; flag any dimension that needs a second check.
Add areas, not lengths
Total area = length A × width A + length B × width B. Alternatively, calculate the surrounding rectangle and subtract the missing corner. Both methods should agree when the sketch and measurements are consistent.
Separate area from the order
The total describes the surface, not the final purchase quantity. Layout, cut pieces and breakage can change the order. Keep the allowance as a separate, editable assumption.