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

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Blog entry by Don W posted 1021 days ago 1848 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.

-- There is nothing like the sound of a well tuned hand plane. - (timetestedtools at hotmail dot c0m)

3 comments so far

View Tedstor's profile


1369 posts in 1133 days

#1 posted 1020 days ago

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


18613 posts in 2661 days

#2 posted 1020 days ago

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


4028 posts in 1789 days

#3 posted 1020 days ago

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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