Given a planar convex set C, we give sublinear approximation algorithms to determine approximations of the largest axially symmetric convex set S contained in C, and the smallest such set S′ that contains C. More precisely, for any var epsilon>0, we find an axially symmetric convex polygon Qsubset ofC with area |Q|>(1−var epsilon)|S| and we find an axially symmetric convex polygon Q′ containing C with area |Q′|<(1+var epsilon)|S′|. We assume that C is given in a data structure that allows to answer the following two types of query in time TC: given a direction u, find an extreme point of C in direction u, and given a line ℓ, find C∩ℓ. For instance, if C is a convex n-gon and its vertices are given in a sorted array, then TC=O(logn). Then we can find Q and Q′ in time O(var epsilon−1/2TC+var epsilon−3/2). Using these techniques, we can also find approximations to the perimeter, area, diameter, width, smallest enclosing rectangle and smallest enclosing circle of C in time O(var epsilon−1/2TC).