Math

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Triangle Perimeter Calculator.

Calculate a triangle perimeter from its three sides.

On-device calculationNo signup
01

Set your values

Results update as you type.

Perimeter: 12

Perimeter

0.000000
sideA: 3

sideA

0.000000
sideB: 4

sideB

0.000000
sideC: 5

sideC

0.000000
Semiperimeter: 6

Semiperimeter

0.000000

Results update automatically as you type.

Use Cases

School Math Homework

Students can quickly verify their manual perimeter calculations for geometry assignments or exam practice.

Example: Check the perimeter of a triangle with sides 5, 6, and 7 units.

DIY and Craft Projects

Crafters and DIY enthusiasts can determine the amount of material needed to frame or border a triangular piece.

Example: Find the total border length for a triangular garden bed with sides 3 ft, 4 ft, and 5 ft.

Frequently Asked Questions

How do I calculate the perimeter of a triangle?
Enter the lengths of all three sides (Side a, Side b, Side c) into the calculator. The perimeter is the sum of these three side lengths. The calculator will automatically add them and display the total perimeter.
What if the side lengths do not form a valid triangle?
The calculator checks the triangle inequality theorem: the sum of any two sides must be greater than the third side. If the entered lengths do not satisfy this condition, the calculator will indicate that the triangle is invalid and you cannot calculate a perimeter.
Can I use this calculator for any type of triangle?
Yes, as long as you have the lengths of all three sides, you can use this calculator for any triangle—equilateral, isosceles, scalene, or even right-angled. Just enter the three side lengths and the perimeter will be computed.

Tips & Common Mistakes

Tips

  • Ensure all side lengths are in the same unit before entering them to get an accurate perimeter.
  • Double-check that the side lengths satisfy the triangle inequality: a + b > c, a + c > b, and b + c > a.
  • If you only know two sides and the included angle, use the law of cosines to find the third side first, then use this calculator.
  • For a quick estimate, round side lengths to the nearest whole number, but remember that the result will be an approximation.

Common Mistakes to Avoid

  • Forgetting to check if the side lengths form a valid triangle, leading to an impossible perimeter.
  • Mixing units (e.g., centimeters and inches) without converting, which gives an incorrect perimeter.
  • Entering negative numbers or zero for side lengths, which are not valid for a real triangle.

Last updated: August 13, 2026