We give a heuristic argument predicting that the number N * (T) of ra-tionals p/q on Cantor's middle thirds set C such that gcd(p, q) = 1 and q ≤ T , has asymptotic growth O(T d+ε), for d = dim C. We also describe extensive numerical computations supporting this heuristic. Our heuristic predicts a similar asymptotic if C is replaced with any similar fractal with a description in terms of missing digits in a base expansion. Interest in the growth of N * (T) is motivated by a problem of Mahler on intrinsic Diophantine approximation on C.