For d ≥ 2 and n ∈ N even, let pn = pn(d) denote the number of length n self-avoiding polygons in Z d up to translation. The polygon cardinality grows exponentially, and the growth rate lim n∈2N p 1/n n ∈ (0, ∞) is called the connective constant and denoted by µ. Madras [J. Statist. Phys. 78 (1995) no. 3-4, 681-699] has shown that pnµ −n ≤ Cn −1/2 in dimension d = 2. Here we establish that pnµ −n ≤ n −3/2+o(1) for a set of even n of full density when d = 2. We also consider a certain variant of self-avoiding walk and argue that, when d ≥ 3, an upper bound of n −2+d −1 +o(1) holds on a full density set for the counterpart in this variant model of this normalized polygon cardinality.