How to Check for Square

Learn how to check for square and correct it using the 3-4-5 rule or a tape measure with this detailed guide for beginner woodworkers.

Checking for square using a tape measure with text overlay


One of the most common questions I get from the readers is about “square” What is square? A lot of the projects and experts tell you to “check for square and correct”. What exactly does that mean?

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Whether you are a budding carpenter or one with experience, checking for square is essential in woodworking. Making sure everything is square will not only improve the overall look, but it will also help you avoid many problems and will strengthen your projects. Not to mention, your guest will notice a crooked shelf or door.

Taking an extra few minutes will ensure everything is square can save you countless hours or more of fixing it later on.

Let’s dive into it today.

What Is Square in Woodworking?

“Square” is a term commonly referred to in carpentry and implies making sure all your faces, edges, and even the ends align correctly. Essentially, you are making sure that every perpendicular corner is 90-degrees.

rectangular box with no top or bottom on a workbench

Building any piece of furniture or project ranging from a simple table to kitchen cabinets or large treehouse essentially involves building a lot of “boxes”. All of these boxes need to be perfectly “square” to be able to work efficiently. If one of the boxes is “out of square” i.e., not perfectly 90-degrees at the corners, it will cascade slowly into other parts of the build and the project will never be perfect.

This is why checking for square is an essential step in woodworking. But before we get into checking for square, let’s talk about what you can do to make things square.


How to Build Things Square

The first step to making sure everything is square is by building carefully to begin with. There are a few simple things to pay attention to while building which will drastically improve the quality of builds and help you get everything nice and square right off the bat –

  1. Ensure all the saw blades are aligned and square.
  2. Cut off the factory ends of the boards before measuring and cutting for your project.
  3. be careful while cutting boards – make sure that the cuts are accurate.
  4. As you are joining be sure to use clamps and other tools to make sure all joints are square.

When building small boxes, this corner clamping jig is one of my favorites.

woman building a small square box using clamps

For larger pieces, I like to simply camp the right angles to the work piece for the best results. You can also use speed squares for this purpose.

Woman building with right angles clamped to workpiece

No matter how careful you are, you may end up with something that is out of square and that’s ok! We can measure and correct for square.

RELATED – 7 Things you can do with a speed square

How to Check for Square

There are a few different ways of checking for square. They involve simple tools like a tape measure or a speed square.

The 3-4-5 Method or Rule

The 3-4-5 method is essentially the Pythagorean theorem method. Remember that Algebra you swore you’d never use in real life? You may need it to build that bookshelf after all. It is relatively simple.

Follow these steps to find that square using these 3-4-5 methods.

  • Using a tape measure, measure three inches from one of the corners.
Woman marking 3
  • With the tape measure, measure four inches from the adjacent side on the same corner. Make another mark.
Woman marking 4
  • Now, measure the distance between the two marks. If it is 5 inches, you now have a perfect 90-degree angle and a square corner.
Woman checking distance between two marks - if it is square

The above steps can also be completed with a speed square or essentially any ruler.

Pro-tip: Keep your measurements within the same units. ie inches, centimeters, millimeters, etc.


Using a Tape Measure

With this method, you measure the 2 diagonals that exist in the box.

  • Use your tape measure and take a measurement from one corner to the one diagonally across from it.
Woman checking the other diagonal of a box using a tape measure to check if it is square.
  • Repeat this process for the remaining two corners.
Woman checking the diagonal of a box using a tape measure to check if it is square.
  • If both measurements match, then the box is a perfect square.

This is my most used method. It is not only quick but also makes it easy to see how to correct the “square” if there are any errors.

How to Correct for Square

So you tried your best to build it square but still ended up having something off-square. To correct that, there are a couple of options depending on your joinery technique –

Pull/Push for Square

This method can be easily used if you have a joint that is freshly glued or has been joined by nails.

  • Once you measure the diagonals, push the opposite corners with the larger diagonal towards each other. This will push out the two adjacent sides and help correct for square.
  • Measure the diagonals again.
  • If still off square, push the longer diagonal corners together.
  • Check diagonals again.
  • Repeat until both diagonals are equal.
Woman pushing two diagonally opposite ends to correct for square

Once square, hold it in place with clamps until all the glue has dried.

Clamp for Square

If you have a joint that is pretty strong and you won’t be able to push it together like in the above method, you can use clamps to push the sides and slowly tighten until everything is square.

woman attaching clamps to correct for square

And that is how you measure and correct for square.

By paying close attention to your boxes and squares, you will be able to take the builds to a whole new level!


My Favorite Tools –

Below are some of my most commonly used tools while building to make sure I get everything square –

More Beginner Woodworking Tips and Tricks –

See lots more tips and tricks in the Woodworking 101 page.

Anika's goal is to inspire and empower beginners with woodworking, DIY, home improvement, and home decor ideas.
She wants everyone to unlock their creative potential and experience the feeling that comes with making something. Nothing feels better better than seeing something and saying "I can make that!"

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  1. You provide great/informative information. Very easy to see the goal you are trying to reach
    Thank you

    1. Anika Gandhi says:

      Thank you so much Bev!

  2. Tami Metts says:

    I love your tshirt “I make pretty things with power tools”. Do you sell them?

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