How to Make Sure a Corner is Square with the 3-4-5 Rule

Download Article

Line up any project perfectly with this simple formula

Co-authored by Mark Spelman and Luke Smith, MFA

Last Updated: April 13, 2024 Fact Checked

Download Article

Squaring a Corner with the 3-4-5 Method

|

Why does the 3-4-5 rule work?

|

Video

|

Expert Q&A

|

Tips

The 3-4-5 rule is a handy and foolproof way to determine if a corner is perfectly square when doing carpentry or other DIY projects, from building a deck to laying tile. And if your corners aren’t square when laying a foundation or building a frame for a deck, your later measurements will be off, too. We’ll show you how to use the 3-4-5 rule to quickly and accurately measure your corners, then we’ll show you how and why it works, so that you can add a handy new tool to your belt.

Things You Should Know

Measure and mark 3 units from the corner along one side of your project, then measure and mark 4 units from the corner along the other side.
Line up a tape measure between your marks and check to see if the length measures 5 units. If it does, the corner is square.
The 3-4-5 rule uses the Pythagorean Theorem (A² + B² = C²) to ensure your corner forms a right triangle with a 90º angle.

Section 1 of 2:

Squaring a Corner with the 3-4-5 Method

Download Article

Use a tape measure to measure 3 units out from the corner of your project, marking the measurement with a pencil.^{[1]
X
Research source} Use feet, meters, or any other unit to make your measurements, so long as you use the same unit each time.
2
Measure 4 units from the corner along the other side. Using the same unit of measurement, measure along the second side and make a mark at 4 units along its length.^{[2]
X
Research source}
- Multiply each number in the 3-4-5 rule by the same amount to scale it up for a larger project (to get a more accurate measurement), or to convert it to different units. For example, try 30-40-50 cm if you’re using the metric system. For a large room, use 6-8-10 or 9-12-15 feet or meters.
Advertisement
3
Measure the distance between your marks—if it's 5, your corner is square. Line up your tape measure between the 2 marks. If the distance is 5 units, your corner is square!^{[3]
X
Research source} If the distance is less than 5 units, your corner is less than 90º. If the distance is over 5 units, your corner has a measurement of more than 90º. Depending on your project, you may need to adjust your measurements or plans, or rebuild the corner to achieve a more precise angle. Use a framing square as a guide when you do this to avoid further mistakes.
- Once you’ve got a square corner, check the project’s other 3 corners to ensure they’re the same.

Section 2 of 2:

Why does the 3-4-5 rule work?

Download Article

The 3-4-5 rule uses the Pythagorean Theorem to make a right angle. If a triangle has sides measuring 3, 4, and 5 units long, 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."^{[4]
X
Research source}
- 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.
- The 3-4-5 rule works with any numbers of the same proportions. For example, finding a triangle with sides that are 6-8-10 or 9-12-15 units long will also give you a 90º angle.
EXPERT TIP

Joseph Meyer
Math Teacher
Joseph Meyer is a High School Math Teacher based in Pittsburgh, Pennsylvania. He is an educator at City Charter High School, where he has been teaching for over 7 years. Joseph is also the founder of Sandbox Math, an online learning community dedicated to helping students succeed in Algebra. His site is set apart by its focus on fostering genuine comprehension through step-by-step understanding (instead of just getting the correct final answer), enabling learners to identify and overcome misunderstandings and confidently take on any test they face. He received his MA in Physics from Case Western Reserve University and his BA in Physics from Baldwin Wallace University.

Joseph Meyer
Math Teacher

Use this visual trick to understand the Pythagorean Theorem. Imagine a right triangle with squares constructed on each leg and the hypotenuse. by rearranging the smaller squares within the larger square, the areas of the smaller squares (a² and b²) will add up visually to the area of the larger square (c²).

Expert Q&A

Search

Add New Question

Question

So, I should measure 3 ft on one side, 4 ft on the other side, and 5 ft for the center to get a square corner?

Mark Spelman
Construction Professional
Mark Spelman is a General Contractor based in Austin, Texas. With over 30 years of construction experience, Mark specializes in constructing interiors, project management, and project estimation. He has been a construction professional since 1987.

Mark Spelman

Construction Professional

Expert Answer

Yes. Your longest side (hypotenuse) should measure 5 feet from point to point.

Thanks! We're glad this was helpful.
Thank you for your feedback.
If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow
Yes No

Not Helpful 39 Helpful 87
Question

What is the square footage of a 30 x 40 building?

Mark Spelman
Construction Professional
Mark Spelman is a General Contractor based in Austin, Texas. With over 30 years of construction experience, Mark specializes in constructing interiors, project management, and project estimation. He has been a construction professional since 1987.

Mark Spelman

Construction Professional

Expert Answer

The square footage of a 30 foot x 40 foot building is 1,200 square feet.

Thanks! We're glad this was helpful.
Thank you for your feedback.
If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow
Yes No

Not Helpful 51 Helpful 68

Ask a Question

200 characters left

Include your email address to get a message when this question is answered.

Submit

Video

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.

Thanks
Helpful 249 Not Helpful 107
The larger your unit of measurement, the more accurate your end result will be.

Thanks
Helpful 1 Not Helpful 0