Perimeter of a Triangle Calculator

Created by Hanna Pamuła, PhD
Reviewed by Bogna Szyk and Adena Benn
Last updated: Nov 17, 2022

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 a triangle's perimeter or are curious about the formulas for a perimeter of a triangle behind this calculator, keep reading. Check out our other handy tools: triangle area calculator, right triangle calculator, and equilateral triangle calculator – 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 uncomplicated. Just add up the lengths of all the sides of the triangle, and you will obtain the perimeter value:

perimeter of a triangle given 3 sides
  • Formula given three sides (SSS)
perimeter=a+b+c\quad\text{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 you know about the triangle:

perimeter of a triangle given 2 sides and angle between them
  • 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)\begin{split}\text{peri}&\text{meter} = a+b\\ &+\sqrt{a^2\!+\!b^2\!-\!2\!\cdot \!a \!\cdot \!b\!\cdot\!\cos(\gamma) } \end{split}
perimeter of a triangle given 2 angles and side between them
  • Two angles and a side between them (ASA)

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

perimeter ⁣= ⁣a ⁣+ ⁣asin(β) ⁣+ ⁣sin(γ)sin(β+γ)\begin{split} \text{peri}&\text{meter} \!=\! a\!+\!a\frac{\sin(\beta)\!+\!\sin(\gamma)}{\sin(\beta+\gamma)} \end{split}

🙋 For an explanation of the law of sines and cosines, don't hesitate to visit our dedicated calculators: the law of cosines calculator and the law of sines calculator!

How to use our perimeter of a triangle calculator?

Let's take a practical application case as an example. Imagine that you want to make a small garden in your backyard and you want to calculate how much fence you need to enclose it.

  1. Choose the formula, according to the data given. Assume that we know two sides of the triangle garden and the angle between them.
  2. Enter the values into the proper boxes. Dimensions of our triangular garden are equal to a=8 fta= 8\ \text{ft}, b=6 ftb = 6\ \text{ft} and the angle between them is γ=75°\gamma = 75\degree. Remember that you can choose the unit by clicking on its name.
  3. Tadaaaam! The perimeter of a triangle calculator displays the solution in a blink of an eye! In our case, we need 22.67 ft22.67\ \text{ft} of fence.

We are sure that after this detailed explanation and example, you understood what the perimeter of a triangle is. Keep practicing!

Hanna Pamuła, PhD
▲ 3 sides
perimeter of a triangle given 3 sides

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