3-4-5 square check
Enter the two legs of a corner. This works out the diagonal that proves it square, and checks a measured one against it.
in
= 72"
in
= 96"
in
Check it against what square should be
The diagonal should be
120"
A bigger triangle, same corner
A longer diagonal makes the same measuring error worth a smaller angle, so for a full building corner or a deck frame it is worth marking further out than the minimum.
- Leg A
- 96"
- Leg B
- 128"
- Diagonal3-4-5 × 32
- 160"
About checking for square
- Why does 3-4-5 prove a corner is square?
- Because it's a Pythagorean triple: 3² + 4² = 5², so a triangle with those side lengths is guaranteed a right angle opposite the longest side. Lay one leg 3 units along one line and the other 4 units along the other, and the two ends are exactly 5 units apart only when the corner between them is 90°.
- Can I use any multiple, like 6-8-10 or 9-12-15?
- Yes — scaling every side by the same number keeps the triangle similar, so it is still a right triangle. Bigger multiples are usually better in the field: the same 1/8" of tape error is a smaller fraction of a 20' diagonal than a 5' one, so it proves the corner more precisely.
- How close does the diagonal need to be?
- An eighth of an inch is about what a tape measure and a chalk line can hold outdoors, and closer than that is chasing precision the tools cannot deliver. For a small box or a picture frame indoors, aim tighter — a sixteenth is achievable on a bench.
- What if I can't fit a 3-4-5 triangle in the space?
- Use the biggest one that fits, even if the legs aren't whole numbers — the triangle only needs to be a right triangle, not exactly 3-4-5. Measure the two legs you actually used, work out what the diagonal should be, and compare.
You might also need
- Wall framing stud countField studs at 16" or 24" on centre, corner studs, and the king and jack studs every opening adds.
- Deck and fence takeoffBoards, joists, posts, pickets, screws and bags of concrete, with the gaps between boards properly counted.
- Miter angle calculatorThe miter for baseboard, casing or chair rail at any corner angle — flat trim, so there's no bevel to set.