Formula to find segment areas

Can anyone supply a **formula** to assist with the following problem please:

I need to work out the areas of segments of different circles (with known radii and with known perpendicular distances from the midpoint of the chord to the segment-part of the circle). I don't know the length of the chord or the angle of the sector that would be formed if lines were taken from the centre of the circle to the corners of the segment.

Hopefully this makes sense! Many thanks in advance. K