This paper proposes an alternative measure for the slack of the aggregate labor market. The natural rate of hours holds valuable information about the state of the labor market that is not reflected by conventional measures, such as the equilibrium rate of unemployment, because it takes the intensive margin into account and is robust to variations in labor force participation. We set up and estimate a multivariate unobserved-components model using information on GDP, inflation, and hours worked, and apply it to the United States and Germany. The estimated hours gap outperforms conventional unemployment gap measures in a Taylor rule by formal model comparison.