This article aims to measure market efficiency without an information model. The intuition is that an efficient market leaves no arbitrage opportunities for active traders, so the measure of efficiency (MOE) is the proportion of profits available to passive traders for a given level of transaction costs. It is expressed as a percentage score and defined symmetrically with a measure of inefficiency (MOI). It can be computed sequentially from a price series and a round-trip transaction cost. The measure of efficiency is shown to increase with diversification, reduce in longer time periods, and have an inverse relation with volatility. It is shown to be a leading indicator of price movements on a day-to-day basis and ahead of the financial crisis of 2008.