DIY Basics & Hand Tools · 6 min read · Beginner

How to Use a Mitre Box for 45° & 90° Cuts

Use a simple mitre box and fine saw to make repeatable square and angled cuts in trim, mouldings and light timber.

Key points & steps

Check which guide slots your mitre box provides. Common simple boxes include 90° and opposing 45° slots.
Mark the finished length on the workpiece and identify the waste side before placing it in the box.
Position moulding in the same orientation it will be fitted where the joint geometry depends on its face/back position.
Clamp the mitre box to the bench if the design allows and secure the workpiece so neither can move while sawing.
Place a fine-tooth saw into the correct guide slots and start with light strokes until the blade establishes the cut.
Saw without forcing the blade hard against the plastic/wooden guide slots, as widening the slots reduces future accuracy.
Dry-fit paired mitres before fixing them. Small errors can compound at corners, especially in rooms that are not perfectly square.

A nominal 45° cut only makes a perfect 90° corner when the actual corner is square.
For coving or shaped mouldings, the orientation in the box matters—follow the moulding maker’s cutting method.
Use a sharp fine-tooth saw for decorative trim to reduce breakout.