Oct 11, 2018

How to Divide Any Board into Equal Parts without Fractions or Complicated Math

If you do woodworking and DIYing in inches, a solid understanding of fractions is essential. Being able to calculate that half of 4 1/4" is 2 1/8", or that 1 1/2 + 1 3/16 = 2 11/16" is basic shop math that will keep your projects moving quickly.

But often, bringing fractions into the process is, well, completely unnecessary. Let's say you have a board that you'd like to divide into equal parts. You could measure it, then bust out a pencil, paper, and the calculator app, and eventually have to Google a decimal-to-fraction converter to figure out the size of each section. Then, you'd have to find that crazy number on your ruler, and carefully add the units together to mark out your parts. Or... you could just do this.     

Okay, let's say this is a board from a project I'm working on. I want to divide it into three equal parts.


The board measures 11 3/8" wide. If I divide that number by three, I get 3.79167. In fractions, that's 3 19/24, or, in 4r ruler language, about 3 51/64". Or something. Just thinking about that makes my head hurt. 

The truth is, I don't care what that number is. I just want the width to be divided equally. So, since I can't find that number on my ruler easily, I make this simple move: 


I pivot the ruler until it reads a number I can divide by evenly. I'm looking for multiples of three, because that's how many sections I'm looking for. 12" works great, as does 18". In this case, I chose 15". 


Now, I just make marks at the whole numbers that represent my sections. Making sure my ruler falls exactly at 0" and 15" on the edges, I make a mark at 5", and another one at 10". 


Then, I use a square to extend that line to where I need it, say, for a baseline for cutting dovetails.


There you go. Three even sections. 


Let's try another number. This time, I want four sections. So I:

    1) Rotate my ruler until I find a number easily divisible by 4, such as 16".
    2) Make a mark at 4", 8", and 12"
    3) Extend the lines with a square

If I want seven sections, I'd:

    1) Rotate my ruler to 14" 
    2) Make a mark at 2, 4, 6, 8, 10, and 12"
    3) Extend those line with a square


The actual size of the divisions? Totally irrelevant. All that matters is that they're equal. And dividing a line into equal parts has nothing to do with whether that line is square to the sides. 

In this case, trying to do a bunch of calculations not only opens you up to making more mistakes. It's entirely unnecessary.

Now. Go do good work. 






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Chris on Oct 21, 2018:

Hi KP - You're making a big, and incorrect, assumption here: that the point of doing this is to then rip the board into equal pieces. I never suggested that all in the post, and it doesn't make any sense. If you need 3, 4, 5, etc, equally wide pieces, you can just mark the width or set a fence, and cut them. If you're ripping parts, it doesn't matter how big your original board is. You could cut three 1/2" wide pieces from something that's 48" wide if you wanted to.
The goal here is to deal with a board in which you don't *care* what the size of the divisions are, just that they're equally spaced. This will never be true in a cutting operation. I gave the example in the post of dividing a board for dovetails. This technique could also make equally spaced screws, or biscuits, etc. Or, you could do this to a section of wall where you want to hand several items to equal spacing.

Please keep the conversation here constructive. And double check that your criticism is germane to the topic before busting out the snark.

KP on Oct 21, 2018:

Sure, this makes evenly spaced lines. But when you make the cut, the kerf will change the width of the cut off pieces. You could use your 0 kerf blade, but I lost mine long ago. Or if you are only interested in being accurate to less than the width of your kerf, you're good to go.

Dustin on Oct 15, 2018:

Thank you Chris, I understand now; thanks for your quality advice and articles!

Dustin on Oct 15, 2018:

in picture number four, is the butt of the ruler positioned correctly, or should the corner of the ruler meet the corner of the board? I just want to make sure the pivot for my "rotate action is in the right spot...

BJMRamage on Oct 14, 2018:


Not sure how often I'd need this but hopefully when I do I will remember this tip.

Levi OBrien on Oct 12, 2018:

That is beautifully simple. Thank you.