How the calculation works
The formulas
Shelf count means intermediate boards only; the opening’s top and bottom boards are excluded. After edge allowances and shelf thicknesses are deducted, the remaining clear height is divided into one more gap than the shelf count.
Gap = (opening height − top allowance − bottom allowance − count × thickness) / (count + 1)
Underside(i) = bottom allowance + gap + i × (gap + thickness)
Assumptions to check
- Opening height is the clear inside height. Positions are measured upward from the inside bottom.
- Board spans, loads, fixings, and shelf deflection are not checked.
A worked example
A 2 m opening with four 18 mm shelves and 50 mm top/bottom allowances has five equal clear gaps of 365.6 mm.