How to Use a Combination Square for Accurate Measurements

January 31, 2025

A combination square is one of those tools that sits at the bottom of the toolbox looking like nothing much, and then you use one properly and realise how many of your wonky shelves and gappy mitres were never a skill problem at all. They were a marking problem. If the line is out before you cut it, no amount of being careful with the saw is going to save you.

So here is what the thing actually does and how to get the most out of it.

What a combination square is

It is a steel rule with a head on it that slides along and locks wherever you want. The head gives you a 90 degree face on one side and a 45 degree face on the other, and most of them carry a small spirit vial and a scribing pin tucked into the body. Some sets come with extra heads, a protractor for reading angles off and a centre finder for locating the middle of something round, but the square head and the rule are the core of it and that is all you need to get going.

The sliding head is the part that makes it more useful than a fixed try square, because once you lock the rule at a measurement you can carry that exact measurement around a whole job without touching a tape again.

The five things it does

1. Marking a square line across a board

Press the head flat against the edge of your timber so the rule lies across the face, then run your pencil down the edge of the rule. That is a true 90 degrees off the edge, every time, and it takes about two seconds. Flip to the other face of the head and you have got a 45 for your mitres.

2. Checking something that already exists

Hold the head against one surface and see where the rule falls against the other. If light shows under the rule, the joint has moved or it was never square in the first place, and either way you have found it before you fix anything else to it. This is worth doing on an old frame before you hang a door in it.

3. Scribing a consistent offset

Set the head so the rule sticks out by the distance you want, lock it, then run the head along the edge of the material with a pencil held at the end of the rule. You get a line parallel to the edge for the whole length. Hinge recesses and the edge margins on a set of shelves all come out even this way, and it beats measuring 10 separate marks and hoping.

4. Reading a depth

Sit the head across the top of a hole or a rebate, push the rule down to the bottom, lock it and read it off. Handy when you are matching an existing depth rather than setting a new one.

5. A rough level check

The little vial in the head is not going to replace a proper level and it is not meant to. What it is good for is the quick sanity check on a bracket or a bit of pipe while your level is at the other end of the shed.

The scribing pin nobody uses

That sharp little needle that pushes out of the head is a scribe, and it is there because a pencil line disappears on dark timber, on steel and on anything wet. Scoring the line instead of drawing it also gives your chisel or your knife a start, which is the difference between a crisp shoulder and a furry one.

Check that yours is actually square

This is the bit most people never do and it matters, because a square that has been dropped off a bench is not a square any more, it is just a confident looking piece of steel telling you lies.

Take a board with a known straight edge. Hold the head against that edge and draw a line across the board. Now flip the square over so the head sits on the same edge from the other side, line the rule up with the mark you just drew, and draw a second line. If the two lines sit on top of each other, your square is true. If they open up into a narrow V, the error is half that gap and it is time for a new one.

Where they go wrong

Nearly every bad mark comes from one of these.

The head is not pulled hard against the edge, so the whole reading is off before the pencil moves.
The edge you are registering off is not straight to begin with, which is common on rough sawn timber and on anything that has been sitting in a shed getting damp.
The locking knob is done up like a wheel nut, which bells out the slot in the rule over time and stops the head sitting true.
The rule markings have worn off, so you are reading a measurement that is half guesswork.

Looking after it

Wipe it down after a job so metal filings and sawdust are not grinding away at the rule, keep it somewhere dry so the steel does not go rusty on you, and if the head starts sticking in the slot then a drop of light machine oil will free it up. A 300mm is the size that suits most people, and a 400mm is worth having if you are working on wider boards or squaring up framing.

And if you are not sure which one to get

Come and ask. Somebody who has never picked one up gets the same answer here as a chippy who has owned four of them, and plenty of people who have been doing this for years still get the square check wrong, so it is a good question rather than a daft one.

Shop related products

Combination Square, 300mm. The all rounder, and the one most people end up reaching for.
Supercraft Level Set of 2. For when the vial in the square is not enough.
Supercraft Torpedo Level. Short enough to live in the toolbox with the square.
Supercraft Hi-Vis Tape Measure, 8m. Because you still need one for the long stuff.

Did you know we also stock the 400mm combination square, a hi impact 300mm version, a combination square with a level built into the head, and rafter squares in 180mm and 300mm including the Kincrome aluminium one. They are not all on the website, so give us a ring on (08) 8536 4333 to check what is on the shelf.

The rest of the marking and measuring gear sits with the hand tools.