A 3-4-5 Rule Calculator helps you check whether three measured sides create a right angle. It is useful for construction, landscaping, carpentry, tiling, framing, fencing, and other projects that require a precise 90-degree corner.
Enter the length of Side A, Side B, and Side C. The calculator checks the measurements and tells you whether they form a right triangle.
What Is the 3-4-5 Rule?
The 3-4-5 Rule is a simple method for checking a right angle. It is based on the Pythagorean theorem, which states that the square of the longest side of a right triangle equals the sum of the squares of the other two sides.
Side A2+Side B2=Side C2
In a 3-4-5 triangle, the two shorter sides measure 3 units and 4 units, while the longest side measures 5 units.
32+42=52
9+16=25
Because both sides of the equation equal 25, the angle between the 3-unit side and 4-unit side is exactly 90 degrees.
How the 3-4-5 Rule Calculator Works
The calculator takes three measurements:
- Length of Side A
- Length of Side B
- Length of Side C
For the result to be accurate, enter Side C as the longest side of the triangle. The calculator compares the square of Side C with the sum of the squares of Side A and Side B.
If the values are equal, the calculator confirms that the measurements create a right angle. If they are not equal, the measurements do not form a right triangle.
How to Use the 3-4-5 Rule
Step 1: Mark the Corner
Mark the point where you want to create or check a right angle.
Step 2: Measure Side A
From the corner, measure 3 units along one side and make a mark.
Step 3: Measure Side B
From the same corner, measure 4 units along the other side and make a second mark.
Step 4: Measure Side C
Measure the distance between the two marks. If the distance is exactly 5 units, the corner forms a right angle.
Side A=3
Side B=4
Side C=5
Example Calculation
Suppose the three side measurements are:
- Side A: 6 feet
- Side B: 8 feet
- Side C: 10 feet
Check the equation:
62+82=102
36+64=100
100=100
Since both sides are equal, the three measurements form a right triangle and create a 90-degree angle.
Scaled 3-4-5 Measurements
You can scale the 3-4-5 Rule for larger projects by multiplying all three measurements by the same number.
Scaled Sides=3k,4k,5k
Common examples include:
- 3-4-5
- 6-8-10
- 9-12-15
- 12-16-20
- 15-20-25
Using larger measurements can improve accuracy when marking large foundations, patios, decks, fences, driveways, and building layouts.
Important Tips
- Use the same unit for all three measurements.
- Enter the longest side as Side C.
- Pull the tape measure tight for accurate results.
- Recheck your measurements before setting posts, framing, or concrete.
- Use larger scaled measurements when working on larger areas.
Frequently Asked Questions
What is the 3-4-5 triangle rule?
The 3-4-5 triangle rule is a quick way to confirm a right angle. When one side measures 3 units, another measures 4 units, and the diagonal between them measures 5 units, the angle is 90 degrees.
Can I use measurements other than 3, 4, and 5?
Yes. Any scaled version of 3-4-5 works, including 6-8-10, 9-12-15, and 12-16-20. Multiply all three numbers by the same factor.
Which side should I enter as Side C?
Enter the longest side as Side C. In a right triangle, Side C is the hypotenuse, which is the side opposite the 90-degree angle.
Can I use feet, metres, inches, or centimetres?
Yes. You can use any unit of measurement, as long as all three side lengths use the same unit.
What does it mean if the calculator says the triangle is not a right triangle?
It means the three measurements do not satisfy the Pythagorean theorem. Check your measurements and make sure the longest side is entered as Side C.
Why is the 3-4-5 Rule useful in construction?
The rule provides a practical way to set out square corners without specialised equipment. It is commonly used for foundations, decks, walls, fences, patios, and other layout work.
Does the 3-4-5 Rule work for large projects?
Yes. Use a scaled version, such as 6-8-10 or 12-16-20, to check right angles over larger distances.
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