Use the 3 4 5 Rule to Build Square Corners

One of the challenges when creating corners is getting them square. While your room doesn't need to beperfectly square, it's best to get 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. The 3-4-5 method is also useful for smaller carpentry projects, ensuring that all your parts will fit together as planned.

Steps

Using the 3-4-5 Rule

  1. Understand the 3-4-5 method. If a triangle has sides measuring 3, 4, and 5 feet (or any other unit), it must be a right triangle with a 90º angle between the short sides. If you can "find" this triangle in your corner, you know the corner is square. This is based on the Pythagorean Theorem from geometry: A2 + B2 = C2 for a right triangle. C is the longest side (hypotenuse) and A and B are the two shorter "legs."[1]
    • 3-4-5 is a very convenient measurement to check because of the low, whole numbers. The math checks out: 32 + 42 = 9 + 16 = 25 = 52.
  2. Measure three units from the corner along one side. You can use feet, meters, or any other unit. Draw a mark at the end of three units.
    • You can multiply each number by the same amount and still use this. Try 30-40-50 centimeters if using the metric system. For a large room, use 6-8-10 or 9-12-15 feet or meters.
  3. Measure four units along the other side. Using the same unit, measure along the second side, at – hopefully – a 90º angle to the first. Mark this point at four units.
  4. Measure the distance between your marks. If the distance is 5 units, your corner is square.[2]
    • If the distance is less than 5 units, your corner is less than 90º. Move the sides apart.
    • If the distance is over 5 units, your corner has a measurement of more than 90º. Bring the sides closer together.



Tips

  • This method can be more accurate than using a carpenter's (framing) square, which may be too small to get precise measurements over greater lengths.
  • The larger your unit, the more accurate you get.[3]

Things You'll Need

  • Tape measure
  • Pencil

Related Articles

Sources and Citations

  1. ↑ http://www.finishabasement.com/construction/the_3-4-5_rule.htm
  2. ↑ http://thecraftsmanblog.com/getting-square-with-the-345-triangle/
  3. ↑ http://www.thisoldhouse.com/toh/skill-builder/0,,20220544,00.html

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