Box Dimension Calculator — Internal to External Sizing

Add twice the material thickness to the internal length and width to get the external dimensions. The exact calculation depends on the joint type used at the corners, as some joints affect the visible length of each piece.

For small jewellery and trinket boxes, 4-6mm is typical. Medium boxes suit 6-10mm. Large storage boxes need 12-18mm. The material should be thick enough for your chosen joint type - dovetails need at least 6mm.

This calculator gives both. The "Base (fixed underneath)" figure is the full external footprint — the simplest method, fixed with pins, screws or glue blocks under the four walls, which is why the base thickness is added to the external height. The "Base panel (groove-set)" figure is the quality method: the panel drops into a groove cut into the walls and is sized to the interior plus the groove depth on each side (taken as half the wall thickness, capped at 6mm), minus the expansion gap per side so a solid-wood panel can move without splitting the box.

The "Lid (overlay)" figure given here is a solid capping slab cut to the full external footprint and laid on top — it needs no expansion allowance because it is unconstrained. If instead you float a solid-wood lid inside a frame groove or rebate, size that panel like the groove-set base (interior plus groove depth per side, less a 1-2mm expansion gap at each end) so it can move across the grain. Plywood and MDF lids are dimensionally stable and need no movement allowance.