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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: February 19, 2026 Fact Checked
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Squaring a Corner with the 3-4-5 Method
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Why does the 3-4-5 rule work?
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This article was co-authored by Mark Spelman and by wikiHow staff writer, Luke Smith, MFA. 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. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 1,246,174 times.
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.
Steps
Section 1 of 2:
Squaring a Corner with the 3-4-5 Method
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1 Measure 3 units from the corner along one side. Use a tape measure to measure 3 units out from the corner of your project, marking the measurement with a pencil.[1]XResearch 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]XResearch 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.
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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]XResearch 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.
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Why does the 3-4-5 rule work?
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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.[4]XResearch source 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."
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
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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 42 Helpful 91
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 54 Helpful 72
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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 251 Not Helpful 107
The larger your unit of measurement, the more accurate your end result will be. Thanks Helpful 1 Not Helpful 0
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Co-authored by: Mark Spelman Construction Professional This article was co-authored by Mark Spelman and by wikiHow staff writer, Luke Smith, MFA. 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. This article has been viewed 1,246,174 times. 33 votes - 96% Co-authors: 23Updated: February 19, 2026 Views: 1,246,174 Categories: Walls and CeilingsArticle SummaryX
To build square corners with the 3-4-5 rule, first measure 3 units from the corner on 1 side. Turn in a perpendicular direction from the first line and measure 4 units. Then, measure the diagonal between the ends of your 2 lines. If it measures 5 units, your corner is square. If it's less than 5 units, your corner is less than 90 degrees; if it's greater than 5 units, your corner angle is too large. For more tips and an explanation of the math behind the 3-4-5 rulel, read on!Did this summary help you?YesNo
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Reader Success Stories
Anonymous
Oct 22, 2025
"it helped me build a square house."
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Yes No Advertisement Cookies make wikiHow better. By continuing to use our site, you agree to our cookie policy. Co-authored by: Mark Spelman Construction Professional Co-authors: 23Updated: February 19, 2026 Views: 1,246,17496% of readers found this article helpful. 33 votes - 96% Click a star to add your vote
Anonymous
Oct 22, 2025
"it helped me build a square house."
Mick Fowles
Jun 20, 2016
"I had to build a brick wall in a trench at right angles to an existing wall, and this system helped me obtain a right angle and build it with confidence. "..." more
Ross Grenz
Mar 20, 2019
"I was always confused by the 3-4-5 rule, but your site made it easy to understand. Thank you, I will use it on an upcoming project."..." more
Ian Burnby
Apr 27, 2016
"Building a patio in the middle of my lawn and want it to be square. I couldn't remember if it was 2/3/4 or 3/4/5. Thanks."..." more
Manny Garcia
Mar 28, 2017
"Very helpful. I heard about this, but not sure how or if it was an actual working thing. It works." Share yours! More success stories Hide success stories
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Download Article Line up any project perfectly with this simple formula Co-authored by Mark Spelman and Luke Smith, MFA 
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Co-authored by: Mark Spelman Construction Professional This article was co-authored by Mark Spelman and by wikiHow staff writer, Luke Smith, MFA. 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. This article has been viewed 1,246,174 times. 33 votes - 96% Co-authors: 23 Updated: February 19, 2026 Views: 1,246,174 Categories: Walls and Ceilings Article SummaryX
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1 Measure 3 units from the corner along one side. Use a tape measure to measure 3 units out from the corner of your project, marking the measurement with a pencil.[1] 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] 
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] 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. 
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.[4] 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." 
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.