Subtract the x-coordinate of the first point from the x-coordinate of the second point. For example, if you have coordinates of (1,1), (1,4) and (5,1), subtract 1 from 1 to get 0.
Subtract the y-coordinate of the first point from the y-coordinate of the second point. In this example, subtract 1 from 4 to get 3.
Square the results of Steps 1 and 2. In this example, square 0 to get 0 and square 3 to get 9.
Add the two squares. In this example, add 0 plus 9 to get 9.
Calculate the square root of the result to find the distance between the first two points. In this example, take the square root of 9 to find that the distance between point 1 and point 2 equals 3.
Repeat Steps 1 through 5 to find the distance between the second and third point and the third and first point.
Add the distance between the first and second point, the second and third point and the third and first point to find the perimeter of the triangle. In this example, you would add 3 plus 5 plus 4 to find the perimeter of the triangle with points (1,1), (1,4) and (5,1) to be 12.