Schedule Duration Calculator

A man-hour is the unit of work content: one worker producing for one hour. Converting that stock of work into a duration means dividing by the rate at which the crew consumes it, which is crew size multiplied by paid hours per day. The productivity factor sits in front of that division because paid hours and productive hours are not the same. Field studies of construction sites routinely find direct work time between 40 and 60 percent of the shift, with the rest absorbed by material handling, travel, waiting for instruction, tool time, and rework. Estimating databases already embed part of that loss in their published output rates, so the factor entered here, commonly 75 to 90 percent, represents the additional site-specific inefficiency layered on top. The conversion from work days to calendar days assumes only that non-work days are spread evenly through the week, so a five-day week multiplies work days by 1.4. A weather contingency near 10 percent is a temperate-climate default; earthwork and concrete in a freeze-thaw region can lose 15 to 20 percent of scheduled days. The main limitation is that this is a resource-limited estimate for a single continuous activity, not a network schedule. Real projects have sequential dependencies between trades, so total duration usually exceeds the man-hour arithmetic by a wide margin. Adding crew also does not scale linearly: once workers exceed the available work face, output per worker falls, and the Business Roundtable documented in 1980 that sustained scheduled overtime erodes efficiency after a few weeks.

Estimating labor by hours of work content grew out of the scientific management movement. Frederick Winslow Taylor's time studies in the 1880s and 1890s and Frank and Lillian Gilbreth's motion studies of bricklaying in the early 1900s established that a trade task could be decomposed into measured elements and assigned a standard time. Henry Gantt's bar chart, developed around 1910 and used widely in wartime shipbuilding, gave those durations a graphical form that could be tracked against calendar dates. Together these two ideas, measured work content and a plotted timeline, underlie every construction schedule since. The formal machinery arrived after the Second World War. In 1957 James Kelley of Remington Rand and Morgan Walker of DuPont developed the Critical Path Method to schedule plant turnarounds, and in 1958 the US Navy Special Projects Office produced PERT for the Polaris missile program. Both required durations as inputs, which pushed contractors toward systematic man-hour databases. Robert Snow Means began publishing construction cost data in 1942, pairing each work item with a crew composition and a daily output, and comparable labor factor tables were later issued by trade associations for mechanical and electrical work. Commercial scheduling software followed, with Primavera founded in 1983, but the underlying conversion from man-hours to crew days has not changed.