A combined computational and theoretical method is developed to predict the equilibrium ring-chain distribution of the products of ring-opening metathesis polymerization (ROMP). In ring-chain equilibria, the free energy change of the reaction includes an entropic cost associated with forming cyclic rather than linear products and an enthalpic cost if ring strain is significant (i.e., for small rings); The entropy change is determined using statistical mechanics (based on the Jacobson-Stockmayer theory). The enthalpy change is computed with a Monte Carlo configurational search using molecular mechanics (MM3). Using the resulting equilibrium constants, the calculated distribution of cyclic and linear products is in good agreement with experiment. To characterize the polymerizability of cycloolefins, we compute the critical concentration [M](c,infinity), defined as the total monomer lost to cyclic products.