With our perimeter of a triangle calculator you can easily calculate the perimeter of that figure. The tool has the basic formula implemented - the one assuming you know all three triangle sides. But that's not all - our calculator is better than the other ones you can find on the Internet, because we've also implemented two other formulas for triangle perimeter, depending on the values you know. Isn't it awesome?
If you are still wondering how to find the perimeter of a triangle or you are curious about the formulas for a perimeter of a triangle behind this calculator, keep reading. Check out our other handy tools: triangle area, right triangle and equilateral triangle calculators - these are a safe bet for your geometry problems.

What is the perimeter of a triangle?

The perimeter is a distance around the shape - in our case, around the triangle. You can think about it as a path surrounding this figure. In real life problems, a perimeter of a triangle may be useful in making a fence around the triangular parcel, tying up a triangular box with ribbon or estimating the lace needed for binding a triangular pennant. However, we guess that you will probably use it in your Maths class ;)

How to find the perimeter of the triangle? The formula for a perimeter of a triangle

The basic formula is really uncomplicated. Just add up the lengths of all of the triangle sides and you obtain the perimeter value:

  • Formula given three sides (SSS)

    perimeter = a + b + c

However, you don't always have three sides given. What can you do then? In these cases, other equations derived from trigonometry may be used, depending on what is known about the triangle:

  • Two sides and the angle between them (SAS)

    Use the law of cosines to find the third side and then the perimeter:

    perimeter = a + b + √(a² + b² - 2 * a * b * cos(γ))

  • Two angles and a side between them (ASA)

    Use the law of sines to find remaining two sides and then the perimeter:

    perimeter = a + (a / sin(β + γ)) * (sin(β) + sin(γ))

Hanna Pamuła, PhD student

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Perimeter of a Triangle. Calculator | Formula | Definition