dw's shop tips #1: 3-4-5 rule.

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Blog entry by Don W posted 07-02-2011 02:38 PM 2368 reads 5 times favorited 3 comments Add to Favorites Watch
no previous part Part 1 of dw's shop tips series Part 2: Threaded inserts? Really. »

You want to square your project. Its to big or to small for a square. Maybe you can’t reach one corner to do a corner to corner comparison. Use the 3-4-5 rule.

The rule state simply if one side of a triangle is 3 and the other side is 4, the diagonal measurement will be 5. This can be inches, feet, millimeters, miles, arms lengths, anything as long as they are all the same.

Any set of numbers divisible by these will work. Need longer, double them for 6 – 8- 10.

Use the rule in conjunction to double check yourself.
Use it to check your square.
Use it to make your new “antique” wooden square.

-- Master hand plane hoarder. -

3 comments so far

View Tedstor's profile


1610 posts in 1725 days

#1 posted 07-02-2011 08:06 PM

One of the most useful formulas around. I’ve done a couple of flooring proects in my home. Since I don’t own a 16 foot framing sqaure, the ole 3-4-5 rule was invaluable.

View MsDebbieP's profile


18615 posts in 3253 days

#2 posted 07-03-2011 01:28 PM

the first time I saw that used to square up a deck, I was “in awe”. Of course, I always forget what the numbers are and have to do a little math in my head :)

A great tip!

-- ~ Debbie, Canada (

View bigike's profile


4047 posts in 2381 days

#3 posted 07-03-2011 06:08 PM

Thanks for the reminder. I’m gonna have to use this on my next box or picture frame. THANKS again.

-- Ike, Big Daddies Woodshop,

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