Siding is sold by the square — 100 square feet — and boxed one or two squares at a time. Wall area minus openings is the starting figure, but on siding the trim package is nearly as significant as the field material.
Deduct openings, but not aggressively
Subtract doors and large windows. Leave small windows in the count — the offcuts around a small opening are rarely reusable, so the material is consumed anyway.
Gable ends are triangles: length times height divided by two. They also waste heavily because every course is a different length with two angled cuts.
Trim is the hidden cost
Vinyl siding needs J-channel around every window and door, corner posts at every outside and inside corner, starter strip along the bottom of every wall, and utility trim under the soffit.
Count linear feet of each and order separately. This is the part people forget, and it is what sends them back to the supplier mid-job.
Vinyl has to hang loose
Vinyl expands and contracts significantly with temperature — up to half an inch over a twelve-foot panel. Nails go in the centre of the slot and are left about a sixteenth of an inch loose so the panel can slide.
Nailing vinyl tight is the most common installation error. It buckles in the first hot spell and cannot be fixed without taking it off.
Starter strip, corners and J-channel
The accessories on a siding job are a bigger proportion of the cost than people expect and none of them come out of a square-foot calculation.
Starter strip runs the full perimeter at the bottom. Outside and inside corner posts run the full height of every corner. J-channel goes around every window, every door, and along the top of every wall that meets a soffit.
Measure each of those as its own linear-foot total. On a house with a lot of windows, the trim can approach the cost of the siding itself.
Where to start the first course
The first course sets everything above it, and it has to be level rather than parallel to the foundation — which on most houses is not the same thing.
Snap a level chalk line all the way round from the lowest corner of the house, and hang the starter strip to it. A first course that follows an out-of-level foundation gets visibly worse with every course above.
The other decision at the bottom is where the courses will land at the top and at the windows. Working the layout out in advance can avoid a half-width course in the most visible position.