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Thread: 3-4-5 Rule

  1. #1
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    3-4-5 Rule

    Just something Dad taught me and I use from time to time on projects, like building decks, laying out walls, or squaring up outside projects. I thought maybe some of you may not have ever heard of it or been exposed to it. The 3-4-5 rule is used to get things laid out square at a 90 degrees.

    How it works, is you measure out from the inside or outside corner in one direction and make a mark at 3', then measure out from the same corner the other direction 4'. Now measure from the 3' mark to the 4' mark moving one side in or out until the distance between the marks is 5' exactly, this will be 90 degrees.
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    You can increase the accuracy for larger projects by multiplying the figures in the rule (i.e. 3-4-5 x 2 = 6-8-10 or 3-4-5 x 3 = 9-12-15). Also it doesn't have to be feet, use inches or centimeters, or even yards if you've got a tape long enough. The math guys call this Pythagoras' theorem.
    Darren

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  2. #2
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    This rule was derived from Pythagorean's Theory for right (90°) triangles whereby the hypotenuse squared = the sum of each of the other two legs squares, i.e. C2=A2+B2. As Darren pointed out, this is a handy way to set up, or confirm, a square corner for many applications.
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  3. #3
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    Quote Originally Posted by Al Launier View Post
    This rule was derived from Pythagorean's Theory for right (90°) triangles whereby the hypotenuse squared = the sum of each of the other two legs squares, i.e. C2=A2+B2. As Darren pointed out, this is a handy way to set up, or confirm, a square corner for many applications.
    ?HUH?????
    Jon

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  4. #4
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    Quote Originally Posted by Darren Wright View Post
    Just something Dad taught me and I use from time to time on projects, like building decks, laying out walls, or squaring up outside projects. I thought maybe some of you may not have ever heard of it or been exposed to it. The 3-4-5 rule is used to get things laid out square at a 90 degrees.

    How it works, is you measure out from the inside or outside corner in one direction and make a mark at 3', then measure out from the same corner the other direction 4'. Now measure from the 3' mark to the 4' mark moving one side in or out until the distance between the marks is 5' exactly, this will be 90 degrees.
    Click image for larger version. 

Name:	square-joists-00-x.gif 
Views:	174 
Size:	16.5 KB 
ID:	74628
    You can increase the accuracy for larger projects by multiplying the figures in the rule (i.e. 3-4-5 x 2 = 6-8-10 or 3-4-5 x 3 = 9-12-15). Also it doesn't have to be feet, use inches or centimeters, or even yards if you've got a tape long enough. The math guys call this Pythagoras' theorem.
    I think that is the one thing I actually use from tenth grade geometry.

    Ike

  5. #5
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    Quote Originally Posted by Wayne Eikenberry View Post
    I think that is the one thing I actually use from tenth grade geometry.

    Ike
    I think I learned it long before I was even out of elementary school helping dad layout for new foundations, only knew of it as the 3-4-5 rule. I think it was even after HS before I correlated the relationship between geometry and it.
    Darren

    To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault

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    now i know why it never worked for me i always added the numbers and got 7
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    Quote Originally Posted by Jonathan Shively View Post
    ?HUH?????
    It's math talk Jon. Here, I'll translate:

    Put a mark over here at 3 feet. Put another mark over yonder at 4 feet. When you move the two marks 5 feet away from each other, it's 90 degrees outside and it's getting too danged hot to work, so you go get a tall glass of lemonade and think of taking the family to the zoo to go look at all the animals. You know, the giraffes, lions, the square hippopotenuses, and so on. It's all pretty simple.
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  8. #8
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    Quote Originally Posted by Vaughn McMillan View Post
    It's math talk Jon. Here, I'll translate:

    Put a mark over here at 3 feet. Put another mark over yonder at 4 feet. When you move the two marks 5 feet away from each other, it's 90 degrees outside and it's getting too danged hot to work, so you go get a tall glass of lemonade and think of taking the family to the zoo to go look at all the animals. You know, the giraffes, lions, the square hippopotenuses, and so on. It's all pretty simple.
    I was following along with the marks over yonder, but the concept of people paying to see animals???????? Why would someone do that????
    Jon

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  9. #9
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    Quote Originally Posted by Jonathan Shively View Post
    I was following along with the marks over yonder, but the concept of people paying to see animals???????? Why would someone do that????
    So we don't have to deal with the poo.
    “When love and skill work together, expect a masterpiece.” - John Ruskin
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  10. #10
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    You guys really have a wonderful sense of humor. Don't lose it.
    Thoughts entering one's mind need not exit one's mouth!
    As I age my memory fades .... and that's a load off my mind!

    "We Live In The Land Of The Free, Only Because Of The Brave"
    “The problems we face today are there because the people who work for a living are outnumbered by those who vote for a living."
    "
    Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery." Winston Churchill

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