Why use mitre joints on a box?

Mitre joints hide the end grain at corners, giving a clean, elegant appearance. The grain appears to wrap around the corners. They are ideal for decorative boxes, especially when the wood has attractive grain patterns.

How do I strengthen mitre joints?

Mitres can be reinforced with splines (thin strips of wood across the joint), biscuits, dowels, or simply with glue on well-fitted joints. For small boxes, PVA glue alone on accurately cut mitres is often sufficient.

What is the best way to cut accurate mitres?

Use a mitre saw set to exactly 45 degrees, a table saw with a mitre gauge, or a shooting board with a 45-degree fence. Test the angle by joining four offcuts - they should form a perfect 90-degree corner.

WoodCalcs

Mitre Box Calculator — Mitre Joint Box Dimensions

Work out mitre joint dimensions and material lengths for box corners with perfect 45-degree mitres.

Internal Length

Internal Width

Internal Height

Material Thickness

How We Calculate This

This calculator determines the material lengths for a box with mitred corners, where each corner joint is cut at 45 degrees.

How it works

For a four-sided mitre box, each piece is measured to the outer dimension (point to point on the mitres). The external dimension equals the internal dimension plus twice the material thickness.

Key measurement

External length = Internal length + (2 x material thickness)

Frequently Asked Questions

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Last updated: February 2026

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