How to Use a Speed Square | Mark, Cut, and Check Angles

A speed square marks 90° and 45° lines, guides circular saw cuts, and lays out any angle from 0° to 90° using its pivoting corner.

If you have ever watched a framer pull a small triangular tool from a pouch and scribe a perfect cut line in seconds, that is the speed square. The lipped edge hooks the board, and a few simple maneuvers cover almost every marking job on dimensional lumber.

What a Speed Square Does

This triangular layout tool serves three main jobs: marking square cuts, laying out 45° miters, and setting arbitrary angles using the pivot point. It also doubles as a saw guide for straight circular-saw cuts.

The square’s design makes it fast. The lipped fence on one edge grips the board so the tool stays put while you draw. The 90° edge gives you a square line. The diagonal edge gives you a perfect 45°. The pivot point at the corner swings to any other angle.

How to Mark Common Cuts

All marking starts the same way: hook the square’s lip against the board’s edge, hold it tight, and trace along the edge you need.

Square cuts

Place the lipped fence against the board edge with the body lying flat. Press it firm and draw along the 90° edge. That line sits at a right angle to the board’s edge, ready for a crosscut.

45° miter lines

With the fence still tight against the board, trace along the diagonal edge instead. This gives you a 45° line for trim, corner joints, or framing details.

Arbitrary angles

Set the pivot point at the board’s edge, then rotate the square until the degree marking you want aligns with the board edge. Hold it steady and trace along the working edge. This covers any angle between 0° and 90°.

Using a Speed Square with a Circular Saw

The square works as a straight-edge guide for your saw. Hold or clamp the lip firmly against the board, then run the saw’s baseplate along the square’s edge. Keep steady contact through the entire cut.

This method produces clean, straight cuts without setting up a track saw. Keep your fingers clear of the blade path and maintain that stable contact between the shoe and the square.

For checking existing work, place the 90° corner against a board edge. Any visible gap tells you the piece is not square.

Common Mistakes to Avoid

Most speed square errors come down to a few fixable habits. The biggest is failing to keep the lip tight against the board edge — even a small gap throws off your line. Another is using the wrong edge for the job: the 90° edge for square lines, the diagonal for 45°, the pivot for everything else.

Moving the square mid-mark is a third common error. Hold it steady while you trace. When using it as a saw guide, keep the blade shoe pressed firmly against the square the whole way.

One subtle trap involves the angle scale itself. Some roof and pitch markings require thinking in terms of the square’s angle rather than the final cut angle. Verify the markings on your specific model before trusting them for rafter layout.

Parallel Lines and Offsets

The scribing notches along the square’s edge let you draw repeated parallel lines quickly. Hook the notches over the board edge, slide the square along, and mark. This works well for layout work where you need consistent spacing.

The ruler markings on the square’s body also support offset measurements. This is standard speed square function covered in most tool guides, including the guidance from This Old House and Popular Mechanics. When you are ready to pick one up, our roundup of top aluminum speed squares covers tested options for different budgets.