How to calculate the angles of a triangle

All triangles have three angles . If the angles have the same size they will measure all 60 degrees and it will be an equilateral triangle, while a right triangle has an angle of 90 degrees that forms an "L". An angle size can be calculated by measuring the other two angles. Keep reading this article and you will discover how to calculate the angles of a triangle.

Steps to follow:


One of the options you can find is to know two angles of the triangle and have to find the measure of the third angle. Let's take an example: two of the angles of a triangle could be 55 degrees and 25 degrees.


In this way, to be able to calculate the angles of a triangle you will have to add the measure of the two angles that you know. For example, following the case that we have proposed previously, we would have to do this operation:

  • 55 + 25 = 80 degrees, the total of the two angles measured


Now you must subtract the total sum of the two known angles to 180 degrees. Following the previous example, the operation to be performed would be the following: 180-80 = 100 degrees.

In this way, we find as a result that 100º is the measure of the third angle that was the unknown angle.


So that you can understand perfectly how to calculate the angle of a triangle, we will give you another example:

If we know the two angles of a triangle like the one in the image that are 125 and 30 degrees, to find the third angle of the triangle we have to add the known values:

125 + 30 = 155

And then subtract the previous result to 180; therefore the third angle is:

180-155 = 25 degrees


In case it is an isosceles triangle, that is, with two sides and equal angles and the third different one, you can find yourself in two situations:

  • If you only know the different angle, you will have to subtract the measure of this angle at 180º and then divide it by two. For example: 180º - 40º = 140º and 140º / 2 = 70º so that both equal angles measure 70º
  • If you know the size of the equal angles, you must add them and subtract that amount to 180º. Example: 35º + 35º = 70º and 180º - 70º = 110º so that the third angle will measure 110º.


These other articles of:

  • How to measure the angles of the triangles
  • How to calculate the center of a triangle
  • The total measure of the three angles of a triangle equals 180 degrees.