How do I make a straightedge guide for a circular saw?

Building a self-aligning saw guide from two pieces of sheet material so you line the edge up with the cut line rather than measuring an offset.

Difficulty: beginner
Time: 45 min
Read: 4 min
Safety: caution

Short answer

Screw or glue a narrow straight batten onto a wider thin base. Run the saw along the batten once, trimming the base to exactly the width of the saw's baseplate. From then on you line the trimmed edge up with your cut line — no offset, no measuring, no error.

Clamping a batten and measuring the offset from blade to baseplate edge works, but every cut repeats the same opportunity for arithmetic error. A self-trimming guide removes the measurement entirely.

Safety

You will be trimming the guide with the saw itself. Make sure the workpiece is fully supported, the clamps are clear of the blade path, and the offcut cannot pinch the blade and cause kickback. Never hold the guide by hand while cutting — clamp it at both ends.

What you'll need

6mm plywood or hardboard for the base
straight 18mm ply or MDF for the fence
wood glue and short screws
two or more clamps
circular saw

Step by step

Cut a base wider than the saw's baseplate.Thin sheet, 6mm ply or hardboard, and generously wider than the distance from the blade to the outer edge of the baseplate. Length to suit — 1300mm crosscuts a sheet, 2600mm rips one.
Choose a genuinely straight fence.A 100mm wide strip of 18mm ply or MDF with a factory edge. This edge is the accuracy of every cut you will ever make with the guide, so sight along it first.
Glue and screw the fence to the base.Set it back from one long edge by more than the baseplate width. Glue the whole length and add screws from underneath so nothing stands proud on the running surface.
Let the glue cure before trimming.Any movement between fence and base after trimming destroys the accuracy.
Set the saw depth for the base thickness.Just enough to cut the 6mm base cleanly. Setting it too deep means the guide will also score whatever is underneath.
Run the saw once along the fence.Baseplate flat on the base, edge against the fence, full length in one pass. The saw trims the base to exactly its own offset.
Mark the guide clearly.Label which face is up, which edge is the cut edge, and which saw it was trimmed with. A guide is only true for the saw that trimmed it.
Use it by aligning the trimmed edge with the cut line.Place the trimmed edge directly on your marks at both ends. The blade will cut exactly there.
Clamp at both ends, clear of the blade.Clamps must be on the fence side or beyond the ends. A clamp in the saw's path is a serious hazard.
Support the whole sheet.Insulation board or battens across trestles under the whole sheet. An unsupported offcut drops, pinches the blade and causes kickback.
Make a second short one for crosscuts.A 600mm version handles crosscuts on stock and is far easier to handle than a full-sheet guide.

Tips

The trimmed edge is the cut line. That is the entire point — no offsets, no arithmetic, no error.
Wax or a rub of candle on the underside of the base makes the saw glide and reduces the tendency to drift off the fence.
Make one long enough to rip a full sheet and one short enough to crosscut. Two guides cover essentially all sheet work.

Common mistakes

Using the guide with a different saw — The trimmed edge encodes one specific saw's blade-to-edge offset. Another saw, or the same saw with the blade on the other side, cuts in a different place.
Letting the saw drift off the fence mid-cut — The trimmed edge stops matching the blade position for that part of the cut, and the line wanders. Keep firm sideways pressure against the fence throughout.

If it doesn't work

Cut is not where the trimmed edge was

Cause: Saw drifted off the fence, or a different saw was used — Fix: Maintain constant sideways pressure against the fence, and label the guide with the saw it belongs to.

Guide flexes and bows in the middle

Cause: Fence not stiff enough or base too thin over a long span — Fix: Use a deeper fence section, and support the guide along its length rather than only at the ends.

Cut wanders at the start

Cause: Saw not squarely against the fence before the blade engages — Fix: Start the saw with the baseplate already flat and hard against the fence, clear of the work, then feed in.

Blade binding partway along

Cause: Sheet unsupported and closing on the blade — Fix: Support the whole sheet on insulation board or battens, so both sides stay put as the cut opens.

Questions people ask

How is this different from a track saw?

The principle is identical. A track saw adds a splinter guard, a positive location that stops the saw lifting, and better dust extraction. The home-made guide gets you most of the accuracy for almost nothing.

Can I use the same idea for a jigsaw or a router?

Yes, and it works well. Trim a base for each tool separately, because each has a different offset. Label them clearly or they will get mixed up.

What should the fence be made of?

Anything stiff and straight — 18mm ply, MDF, or a straight length of aluminium angle. Avoid solid timber, which moves with humidity.