Carpentry Calculators

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"

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72"96"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.

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