Oct 03, 2016

DIY Tip: How to Quickly Add Fractions for Woodworking Projects... No Math Required

created at: 06/25/2015

You know that old question that some stereotypical student always asks their math teacher? The one that's some variation on "When are we actually gonna use this in real life?" The answer, at least for arithmetic, geometry, and even a bit of trigonometry, is: woodworking. For sure. And plenty of other creative and DIY efforts. (That said, I've never actually used calculus after high school, but I know lots of jobs and creative pursuits require differentials and derivatives all the time. I'm just glad mine doesn't)

Anyone who spends time with a ruler or tape measure gets used to working in fractional inches, and most of us become pretty good with cutting things in half, or adding up thicknesses of different materials. But when your projects and design work start to involve numbers like 7 19/32", things start to get a little difficult to calculate in your head. 

So, here's an easy trick to quickly add complex fractions, with no paper, calculators, or headaches required.

created at: 06/25/2015

Use two rulers. Or two tape measures. Or a framing square and a folding rule. Any two things with a readable scale will do. This is how it works: Let's say we want to add 3  11/16" and 1 5/8". 


created at: 06/25/2015

Find 3  11/16" on ruler A. Then, place ruler B at 3  11/16"and find the second number, 1 5/8", on ruler B. The adjacent graduation on ruler A is the sum. 

See how easy that is? Looking at the photo above, the answer is 5 5/16". 

created at: 06/25/2015

Done. No common denominators required. 

This works for subtraction too. Just read the numbers in the opposite direction. If you do regularly work with complex fractions and are as right brained as I am, a shop calculator is a useful tool. This allows you to add fractions without converting them into decibels, as well as a heap of other tasks. Until we're all on the metric system (which will be a great day, indeed), it can be a lifesaver.

ManMade Recommended: 


created at: 06/25/2015

But! The two ruler technique totally works, and it's free. Who doesn't have a ruler and a tape measure lying around? Do this quick trick, and get back to building stuff. 


Here it is again in simple photos: 

created at: 06/25/2015

Happy making!




Edited from a post originally published in May 2015.



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Brian on Jan 09, 2019:

Thumbs up, Chris! Appreciate the info!

Zs on Feb 10, 2018:

Almost 2.5 yrs later, I'll go ahead and comment my 2 cents: I learned a while ago (30 yrs ago-- 3rd/4th grade... and every yr thereafter) that doing fractions was gonna suck. My 4th grade teacher actually gave me this huge cheat-sheet and it literally was additional tables for fractions. Found it again recently and my uncle (engineer for 1 of the biggest firms in Chicago area) looks at it and says "Hey, I've got the "newer" one of those- it's the top of my desk... laminated on top of the desk"- by new he means 20 yrs old. He is accessing multi-million dollar servers, next gen Auto-cad, and specialty tablet, plus Autocad 360 on his phone, and he's got a legacy item? His response: "Cause it's faster to look at" The SAE system isn't going away anytime as long as there is an industry that still uses it.

bill on Aug 11, 2017:

Cheating, but awesome!

Stephen Roberts on Jul 05, 2016:

What an excellent system! 

Linnea on Mar 03, 2016:

How does one convert fractions to a measure of sound (decibels)? Now that's a real talent ;)

messenger on Feb 09, 2016:

You've essentially made a slide rule for fractions. Cool. :)

Both metric and imperial have there advantages and disadvantages.

Butch on Oct 28, 2015:

Love your response!

Jacob on Jun 30, 2015:

This is brilliant. Thanks for the tip!

jay on Jun 26, 2015:

Another quick math tip to find the half of any length on a tape measure is to simply bend the tape back on itself and place the tip of the tape measure at the number to be halved.  The number at the bend of the tape is your half!  You can also use this method to subtract numbers.  It's an incredibly simple method for those times when you just can't seem to think right.

Chris on Jun 25, 2015:

Hi Dave - You gave a fake email address with your comment, so I don't know if you'll see this, but for the rest of us: 1) I did address the fact that the metric system makes more sense in the post above (did you read the whole thing before commenting?) and 2) you simply cannot do woodworking in the United States on the metric system. I actually undertstand the metric system quite well and use it regularly. I have no idea where you're based, but in the United States, the tools are on the standard system, lumber is standard, hardware is standard, and all instructions are standard. I was trying to offer a legitimate way to do quick work with the realities of making stuff in the US, where most of our readers are based. No one is going to convert every other aspect of a project to another system of units...that would make everything five times longer than just embracing fractions in the first place. I always welcome conversation, but I owe the readers of this site practical advice that I hope improves their lives and their making process. I'll continue to do that. 

Dave on Jun 25, 2015:

I got one word for you: metric !