Compute the area of the intersection, which is a rectangle too:
SI = Max(0, Min(XA2, XB2) - Max(XA1, XB1)) * Max(0, Min(YA2, YB2) - Max(YA1, YB1))
From there you compute the area of the union:
SU = SA + SB - SI
And you can consider the ratio
SI / SU
(100% in case of a perfect overlap, down to 0%).