Circular segment - is an area of a circle which is "cut off" from the rest of the circle by a secant (chord).
On the picture:
L - arc length
If you know radius and angle you may use the following formulas to calculate remaining segment parameters:
Circular segment formulas
But if you don't know radius and angle you still can caluclate the segment parameters by chord length and segment height:
Formula for segment radius by chord and height:
Then, you can caluclate segment angle using the following formula:
You may also use the following calculator to obtain segment area by its radius and height:
This calculator evaluates angle by the following formula:
then it uses formule  to calculate the segment area.