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?

For quality boxes, the base typically sits in a groove (rebate or housing) cut into the sides, about 6mm up from the bottom. This hides the base edge. For simpler construction, the base can sit underneath, fixed with pins or screws.

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

Solid wood lids should float in a groove or rebate, with 1-2mm expansion gap at each end. For cross-grain construction, allow more movement. Plywood lids do not need expansion allowances.

WoodCalcs

Box Dimension Calculator — Internal to External Sizing

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

Internal Length

Longest dimension of the area

Internal Width

Shortest dimension of the area

Internal Height

Measure floor to ceiling or top of area

Side Material Thickness

Material thickness

Base Thickness

Material thickness

Corner Joint Type

Select the woodworking joint type

Include Lid

Toggle to include or exclude from calculation

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.

Frequently Asked Questions

Related Calculators

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Lid Fit Calculator

Calculate lid dimensions and clearance for tight-fitting box lids.

Hinge Placement Calculator

Determine hinge positions and mortise dimensions for box lids.

Divider Calculator

Calculate divider dimensions and slot positions for compartmented boxes.

Inlay Calculator

Work out inlay recess dimensions and material quantities for decorative work.

Last updated: February 2026

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