definition sharpe ratio is used to measure the return of an investment to its risk. it is a mathematical expression that is a measure of the excess return (above the risk-free rate) earned in excess of what would have been earned on a risk vs reward investing investment during the same period. when employed for evaluating a portfolio, the ratio can be computed for any number of time periods. for example, the ratio may be computed from daily price data to evaluate performance over one year, or from monthly returns to monitor results over three years.