Example 1 — Classic 3-4-5
- Side A: 3
- Side B: 4
- Side C: 5
The layout forms a right angle
This is the basic pattern that gives the method its name.
3-4-5 ratio
Check whether side lengths form a right angle and use the classic 3-4-5 rule for simple on-site layout work.
The 3-4-5 rule is one of the simplest ways to set out a right angle without advanced equipment. This calculator helps you quickly check whether given lengths match a right triangle, or how close they are to the correct proportion. Related: slope percent, stair step height.
Results
Enter data and click Calculate.
How the results are calculated
Checks a² + b² ≈ c² (Pythagorean theorem). With a tolerance in mm, |c − √(a²+b²)| must stay within that tolerance.
How to use this calculator
- Enter the three side lengths.
- Compare the result with the Pythagorean rule or the calculator’s right-angle check.
- See whether the layout forms a right angle or still needs adjustment.
- Use the result when setting out a foundation, wall, terrace, or any rectangular geometry.
Example scenarios
Example 2 — Larger 6-8-10 layout
- Side A: 6
- Side B: 8
- Side C: 10
The layout forms a right angle
The rule works not only for 3-4-5 itself, but for any proportional multiple of those lengths.
Example 3 — Layout close to correct but not exact
- Side A: 3.05
- Side B: 4.0
- Side C: 5.0
The layout is not perfectly square
This is a practical field example where a small deviation shows that the setup still needs adjustment.
FAQ
What is the 3-4-5 rule?
If the sides of a triangle follow the 3, 4, and 5 proportion, the angle between the shorter sides is a right angle. It is a practical application of the Pythagorean theorem.
Can I use numbers other than 3, 4, and 5?
Yes. Multiples also work, for example 6-8-10 or 9-12-15.
What is this method used for on site?
It is used to set out right angles for foundations, walls, terraces, fences, and many other rectangular layouts.
Does a small deviation matter?
In practice, yes. A small error at the start can turn into bigger alignment problems later during construction or installation.
Should the calculator give only a yes/no answer?
Ideally, it should also show how close the layout is to a true right angle. That makes the tool much more useful.
Does this replace professional measurement?
No. It is a fast practical aid, but it does not replace precise surveying or formal construction measurement.
Why is the method so popular?
Because it is simple, inexpensive, and works in real site conditions without complex equipment.
Which calculator is related to this one?
It is closely related to tools for slope, stairs, and roof geometry — anywhere that correct layout and angle matter.