How do you check a corner is square?
Measure the diagonal and compare it to what Pythagoras says it should be. The 3-4-5 triangle, why a bigger one is better, and what a half-inch error actually means.
Measure 3 units along one side and 4 along the other; square, the diagonal between the marks is exactly 5.
Scale it as large as the job allows — on a deck frame, 36" and 48" giving 60". A 12' × 16' frame should measure 240" corner to corner.
How that is worked out
- A triangle with sides 3, 4 and 5 has a right angle between the 3 and the 4. It is the smallest whole-number case of Pythagoras and it has been used on building sites for several thousand years.
- Any multiple works: 6-8-10, 9-12-15, 12-16-20. Use the largest that fits, because a long diagonal turns the same measuring error into a much smaller angular error.
- For a rectangle, check both diagonals instead — they are equal only when it is square. A 12' × 16' frame wants 240" both ways.
Diagonals for a square rectangle
| Rectangle | Diagonal | Nearest sixteenth |
|---|---|---|
| 3' × 4' | 60.00" | 60" |
| 4' × 8' | 107.33" | 107 5/16" |
| 8' × 10' | 153.67" | 153 11/16" |
| 10' × 12' | 187.45" | 187 7/16" |
| 12' × 16' | 240.00" | 240" |
| 16' × 20' | 307.35" | 307 3/8" |
Worth knowing
- 1/8" is about what a tape and a chalk line can honestly hold over a long diagonal, so that is the tolerance the calculator uses. On a cabinet you would expect far better; on a deck frame in the rain, that is realistic.
- Half an inch out on a 12' × 16' frame is outside that tolerance — which shows how quickly a diagonal error grows into something you can see.
- Rack the frame to correct it: push one corner diagonally and both diagonals change in opposite directions. Brace it the moment they agree, before anything else goes on.
- A framing square is only as good as its length. For anything bigger than a cabinet it is checking a few inches of a joint and telling you nothing about the other eight feet.
Work out your own numbers
- 3-4-5 square check
The diagonal that proves a corner square from any two leg lengths, scaled up for a full building corner.
- Deck and fence takeoff
Boards, joists, posts, pickets, screws and bags of concrete, with the gaps between boards properly counted.
- Wall framing stud count
Field studs at 16" or 24" on centre, corner studs, and the king and jack studs every opening adds.
Related questions
- What is the 3-4-5 rule?
- Measure 3 units along one side of a corner and 4 along the other; if the corner is square, the distance between those two marks is exactly 5. The units can be anything as long as they are the same — feet, inches, or arbitrary marks on a stick. It is Pythagoras with the smallest whole numbers that work.
- How do I check a deck frame is square?
- Measure both diagonals. They are equal only when the frame is a true rectangle, and that single check covers all four corners at once. If they differ, push the long diagonal's corner inward until they match, then brace it before you do anything else.
- Why use a bigger triangle than 3-4-5?
- Because the same measuring error means a smaller angular error over a longer distance. Being 1/8" out on a 5-foot diagonal is a much bigger angle than 1/8" out on a 20-foot one. Use the largest multiple your tape and your workpiece allow.