How to Apply the 3 4 5 Rule to Make a Right Angle

One of the challenges when creating corners is making them square. Although the room doesn’t have to be perfectly square, the angles should be as close to 90 degrees as possible. Otherwise, when laying tiles or carpeting, you will notice the deviation right away. The 3-4-5 method is also useful for small carpentry projects and helps all the pieces fit together as intended.

Table of Contents

Apply the 3-4-5 . rule

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You need to understand the 3-4-5 method. If a triangle has sides 3, 4, and 5 cm (or whatever), it’s a right triangle with a 90º angle between the two short sides. If you “see” this triangle at the place to be measured, the angle between the two short sides is definitely right. This is based on the Pythagorean theorem derived from Geometry: in a right triangle, A ² + B ² = C ² . In which, C is the longest side (the hypotenuse), and A and B are the two short sides (or right angle side).

The 3-4-5 method is convenient to check because these are small positive integers. According to the Pythagorean theorem, we have: 3 ² + 4 ² = 9 + 16 = 25 = 5 ² .

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Measure three length units (optional) from the corner to one side. You can use meters, centimeters, or any other unit. Mark at the position the length of the 3 units just measured.

You can multiply each number by the same number of times and it still applies. Try measuring 30-40-50 cm if using the metric system. For larger rooms, you can apply sets of 6-8-10 or 9-12-15 meters. ^{[1] X Research Source}

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Continue measuring 4 units on the second adjacent edge. Use the same units of length and measure the second side where it can be perpendicular to the first side. Mark at the position of length 4 units just measured. ^{[2] X Research Source}

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Measure the distance between two markers. If this distance is 5 units, the angle between the two sides that you marked is a right angle. ^{[3] X Research Sources}

If this distance is less than 5 units, the angle to be determined will be less than 90º. Let’s move the two edges away from each other.
If this distance is more than 5 units, the angle you need to determine is greater than 90º. Let’s move the two edges closer together. You can use a frame or square ruler as a guide to align the angle.
After aligning the right angle, you can check the other three corners of the room to get the same result.

Advice

This method will be more accurate than using a carpenter’s frame or square, as these tools are too small to get accurate measurements in large spaces.
The larger the unit of measure, the more accurate the measurement will be. ^{[4] X Research Sources}

Things you need

Tape measure
Pencil

Steps

Apply the 3-4-5 . rule

Advice

Things you need

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