How this calculator works
Subtract door and window areas, add siding waste and divide by installed panel coverage. Following the cited CertainTeed measuring guidance, the trim allowance includes J-channel around windows and three sides of doors, undersill above and below windows, above doors and at the top course, plus starter along the wall base. Corner posts, eaves, gables, extra receiving channels and trim-stock cutting waste are outside this single-wall estimate.
Formula
Net area = W x H - door count x door area - window count x window area; panels = ceil(net area x (1 + waste)/(usable panel length x installed exposure)); opening J-channel = doors x (2 door heights + door width) + windows x 2(window height + window width).
Worked example
A 5 m by 2 m wall has 10 m2 gross area. Subtract a 1 by 2 m door and a 1 by 1 m window to leave 7 m2. Panels covering 1 m2 each need seven pieces before waste.
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Frequently asked questions
Why use exposed height?
Locks and laps reduce installed coverage below overall panel width. Use the manufacturer exposure or coverage figure to avoid underordering.
Are all windows the same size?
This form repeats one set of window dimensions and one set of door dimensions. Calculate wall sections separately for different opening sizes.
Does the panel count optimise offcuts?
No. It is an area-based estimate with an allowance. A course-by-course cutting plan may need more pieces, especially around many openings.