Make a perpendicular line on the chord, intersect with the arc at point A, and point A is connected with "point B of the left intersection of arc and chord".

Then make a median vertical line on the AB line, which intersects with the previous median vertical line at the O point.

Then the OA line is the radius of the arc. With the radius, the circumference of the circle can be calculated.

2. Calculate the length of AB line and the degree of angle AOB by trigonometric function,

Use the following formula to calculate:

Arc length = circumference of circle x (degree of angle A0B divided by 360)x2.