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>Home >Basement_construction >The 3-4-5 rule

The 3-4-5 rule for squaring corners

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One of the challenges when creating corners is getting them square. While no room is ever PERFECTLY square, we need to get the corners as close to 90 degrees as possible. If not, any tile or carpet laid will be noticeably 'off' from one side of the room to the other. Using the 3-4-5 method for squaring corners, and a framing square will help ensure your corners are square.

To create corners, we use the 3-4-5 rule derived from the Pythagorean theorem of basic geometry: A2 + B2 = C2. This means the square of the hypotenuse of a right triangle is equal to the sum of the square of both legs.


Getting away from the geeky math stuff, it simply means that if you measure 3' out from the corner in one direction, and 4' out in the other direction, the line between the two points should be 5'. If not, your corner isn't square.

You may not think this is a big deal (since most rooms aren't perfectly square anyway). However, over the course of 15-20 feet, it would make a noticeable difference.

If you're tiling the floor, or using a covering (carpet, linoleum, etc.) with a pattern, it will be very noticeable if your corner isn't straight!

diagram showing the 3-4-5 rule

Construction Topics

Tools you'll need

The 3-4-5 rule

How to frame a wall

How to attach walls

How to install insulation

Drywall estimating

How to hang drywall: Step-by-step

How to cut drywall

How to cut around outlets and openings

Drywall installation tips

How to frame a door

How to trim a door to fit

How to finish a door

How to frame around poles

How to finish drywall

How to sand drywall

Tips for buying, finishing, & installing baseboard molding

How to fix squeaky floors


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