This article derives a quarterly series from 1952–1968 on the average workweek of capital in use in manufacturing. The basic information used is data on the number of production workers engaged on first, second, and third shifts and the average workweek on the first shift. The nationwide series is built up from periodic data on SMSA's for which results are also given. The series shows a different cyclical behavior from the Wharton capacity utilization index and a different long-run behavior from the Federal Reserve's index of capacity utilization.