Siding Calculators
Siding is the only cladding on this site where a large share of what you buy is designed never to be seen.
The calculators
Every other material here covers the area you measure. Lap siding does not: it is sold at its face width and covers the wall at its exposure, and the difference is permanently behind the course above. On common products that is 12 to 20% of every board.
It is not waste and no allowance covers it. A calculator that takes wall area and adds ten per cent for cuts has under-ordered by roughly the whole lap, which is how a siding job ends up one elevation short with the scaffold still up.
The drainage plane is the actual product
Siding is not waterproof and is not meant to be. Water gets behind it, and what keeps the wall dry is the barrier underneath and the fact that every layer laps over the layer below it, so anything running down the wall stays outside the assembly.
IRC R703.2 requires that barrier over studs or sheathing on all exterior walls, as one layer free from holes and breaks, and it gives two different laps for felt: two inches between courses and six at joints. Almost no siding calculator counts the barrier at all, let alone its laps.
Shingle-fashion is the whole idea
R703.4 requires flashing applied shingle-fashion, extending to the surface of the exterior wall finish. One lap the wrong way round at a window head turns the drainage plane into a funnel pointing into the wall cavity, and no quantity on any order prevents that. It is the failure mode of the trade and it is a detailing question rather than an arithmetic one.
What varies by product rather than by code
Exposure. The same nominal board is specified at different exposures by different manufacturers, and the maximum is a weather-tightness limit rather than a preference. These pages ask for the figure instead of assuming one, and the number that matters is in the instructions for the siding you actually bought.