This paper develops metrics and methods for quantifying the manipulability of cash-settled derivative contract markets. Many derivative contracts, such as futures, options, and swaps, are settled based on price benchmarks, which are calculated based on trade prices of underlying assets. Derivative contract volume is often much larger than the volume of underlying trade used to construct price benchmarks, so these markets may be vulnerable to manipulation: contract holders may trade the underlying asset in order to move benchmarks and influence contract payoffs. I show that derivative contract markets can be much larger than underlying markets without creating large incentives for manipulation, as long as underlying markets are sufficiently competitive. I show how to estimate manipulation-induced benchmark distortions using commonly observed market data. I propose a simple manipulation index which can be used as a diagnostic metric to detect potentially manipulable contract markets, similar to the Herfindahl-Hirschman index (HHI) in antitrust. I apply my results to study contract market competitiveness using the CFTC Commitments of Traders reports, to measure the manipulability of the LBMA gold price benchmark, and to propose a less manipulable design for the CBOE Volatility Index (VIX).