Dividing the cabinet height by the number of openings gives a number that is always too big, because the shelves occupy height too. Four three-quarter inch shelves take three full inches out of the opening before anything sits on them.
Shelves make one more gap than there are shelves
Four shelves in a cabinet create five spaces: one below the first shelf, three between them, and one above the last. This is the fence-post problem in vertical form and it catches people the same way.
So the arithmetic is the opening height, minus the total thickness of the shelves, divided by the number of shelves plus one.
Get this wrong by one and every shelf lands in the wrong place — which matters because shelf holes are drilled before assembly and are not easily moved afterwards.
What actually needs to fit
Even spacing looks deliberate in a drawing and is usually wrong in use. The heights worth knowing are: paperbacks around 8 inches, hardbacks 10, art books and binders 12, LP records 13, dinner plates standing 11.
A pantry wants one tall opening for cereal boxes and bottles and several short ones for cans, where an evenly divided cabinet wastes most of its height as air above short items.
The usual compromise is graduated spacing — taller openings at the bottom, shorter toward the top. It also looks better, because the eye reads the heavier proportions low down as stable.
Adjustable shelves and the 32 mm system
Drilling a line of holes and using shelf pins converts this from a permanent decision into an adjustable one, and it costs almost nothing at build time.
The cabinet industry standard spaces those holes 32 millimetres apart, which is about 1.26 inches, and every commercial shelf pin, drawer slide and hinge plate is designed around that grid.
Using the same spacing means hardware fits without custom drilling, and a shelf-hole jig for the system costs less than an hour of measuring and marking.
Span, sag and what holds weight
Clear height is one question; whether the shelf holds is another. A three-quarter inch shelf in solid wood or plywood sags visibly past about 32 inches of span with books on it, and particleboard sags well before that.
The cures are a shorter span, a thicker shelf, or a stiffener glued to the front edge. A one-and-a-half inch front edge on a three-quarter inch shelf roughly doubles the stiffness for very little material, because stiffness goes with the cube of the depth.
Sag is slow and permanent. A shelf that has held books for five years does not come back flat when you unload it.
Depth is the other dimension people regret
A 12 inch deep shelf takes a paperback and a hardback. A 16 inch shelf takes art books but also invites two rows of paperbacks, and the back row is then invisible and unreachable.
Kitchen wall cabinets are 12 inches deep almost universally, for the same reason: past that, the back of the shelf stops being usable storage and becomes somewhere things disappear.
Deeper is not more storage if you cannot see what is on it.