Measure the clear path, not the shelf board
A tote has to travel from outside the unit to its resting position. Every measurement should be the narrowest point along that whole path, not the widest point at the resting position.
Uprights, corner posts, front lips, diagonal braces, wire-shelf edge beading, cabinet face frames and door hinges all intrude into the path. Any one of them can be the real limit while the shelf board itself looks generous.
The three numbers to write down
Width: the narrowest side-to-side clearance anywhere along the path, measured between the obstructions, not between the outer frame rails.
Depth: front edge to the furthest usable point. Stop at whatever the tote will actually hit — a wall, a back brace, a cable run, or the point where a rear lip begins.
Height: the vertical gap between the shelf surface and the underside of whatever is above it. On adjustable shelving, measure to the lowest thing overhead, which may be a shelf bracket rather than the next shelf.
Measure twice, at both ends
Shelving units rack slightly out of square once loaded, and older wooden shelving sags in the middle. Measure the height at the front of the shelf and again at the back, and take the smaller value.
If the two readings differ by more than a few millimetres, use the smaller one and consider adding clearance in the calculator.
Check your own measurements
Enter these three numbers as the shelf opening — the calculator runs entirely in your browser and stores nothing.