Building your own shelving, cabinets or a pen from sheet material takes more than a tape measure: you need the bays to stop shelves sagging, the parts for every board, and the number of sheets to buy. These calculators work out the bays, the cut list and the sheets, with a sag check so a full bookshelf stays straight.
They are planning tools for sheet-built carcasses (plywood, MDF, particleboard) and give a starting list for a build, not a structural design. Check wall fixings and anything that carries real load.