How do I calculate external box dimensions from internal?

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.

How thick should box sides be?

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.

Should the base sit inside or under the box?

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.

How do I allow for wood movement in a box lid?

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.

Box Dimension Calculator — Free Box & Case Making Tool

Box Dimension Calculator — Internal to External Sizing

Calculate material dimensions for box sides, base, and lid from your desired internal measurements.

Internal Length

Clear interior length you need (long axis)

Internal Width

Clear interior width you need (short axis)

Internal Height

Clear interior depth from the base up (wall piece height)

Side Material Thickness

Thickness of the four wall pieces (4-6mm small boxes, 6-10mm medium, 12-18mm large)

Base Thickness

Thickness of the bottom panel; added below the walls for the external height

Corner Joint TypeCorner joint — sets how the wall pieces are cut to length

Include Lid

Add an overlay lid (its thickness is added to the external height)

How We Calculate This

This calculator converts your desired internal box dimensions into the material sizes needed, accounting for joint type and material thickness.

Joint type effects

Butt joint: Front/back pieces are cut to internal length; sides overlap to full external width.

Mitre/Dovetail/Box joint: All pieces cut to full external dimension.

Rebate: Front/back at external length; sides at internal width (the rebate is cut into the front/back ends, so the side pieces are sized to the rebate shoulder).

Base, lid and material

The base is given two ways: the full external footprint for a base fixed underneath (its thickness is added to the external height), and a smaller groove-set panel for a base captured in a groove cut into the walls (interior plus groove depth per side, less the expansion gap). The overlay lid caps the full external footprint. The “wall stock to rip” figure adds the four wall-piece lengths plus the saw kerf for the three cuts that separate them, so you can buy the right board length.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.

Box Dimension Calculator — Internal to External Sizing

How We Calculate This

Joint type effects

Base, lid and material

Frequently Asked Questions

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