3 4 5 Triangle Rule

3² + 4² = 5²

A triangle with sides 3, 4, and 5 is always a right triangle.

Builders and designers use the 3-4-5 rule to check perfect right angles quickly.

3 units

First leg

Measure 3 units along one side.

4 units

Second leg

Measure 4 units on the other side.

5 units

Diagonal

If the diagonal is 5, the corner is 90°.

✔How to use the rule

Mark the corner

Start at the corner you want to verify.

Measure 3 and 4

Mark 3 units on one edge and 4 on the other.

Check the diagonal

Measure the distance between the two marks.

Confirm 5

If the diagonal is 5 units, the corner is square.

Yes. Multiply all sides by the same number for larger layouts.

Because it satisfies a² + b² = c², which proves a 90° angle.

Common in construction, tiling, framing, and layout work.

Walk through classic substitutions, squaring, and square-root steps.

Turn ladders, diagonals, layouts, and distance stories into calculator inputs.

Check whether three side lengths make a right, acute, or obtuse triangle.

Use 3-4-5 when you need a quick right angle check.