PERT Calculator

The Program Evaluation and Review Technique, usually called PERT, is a project-planning method for activities whose durations are uncertain. Instead of relying on one date, an estimator supplies an optimistic time, a most likely time, and a pessimistic time. The optimistic value represents a favorable but realistic case, the most likely value reflects normal conditions, and the pessimistic value captures a difficult but still plausible case. PERT combines these values with a weighted average: expected time equals optimistic plus four times most likely plus pessimistic, divided by six. Giving the central estimate four times the weight keeps the result near the duration judged most probable while still recognizing risk at both extremes. The estimates may use hours, days, weeks, or any consistent unit; the output uses that same unit. The standard deviation is the pessimistic estimate minus the optimistic estimate, divided by six. Its square is the variance. A wide range produces greater variance and signals that the task deserves contingency planning, investigation, or decomposition into smaller activities. A narrow range indicates stronger agreement about likely duration, although it does not guarantee that hidden dependencies are absent. Project managers can calculate PERT values for each activity in a network and add expected durations along a path. Variances are commonly added for independent activities, with the square root of the total used as the path standard deviation. The longest expected route through dependent tasks is the critical path. Delays on that path are more likely to move the completion date than delays on activities with schedule float. PERT is only as sound as its inputs. Teams should define what completion means, use historical delivery data, and discuss assumptions before selecting estimates. The classic method approximates activity durations with a beta distribution and often assumes independence when combining variances. Real projects may contain correlated risks, resource constraints, rework, and changing scope, so do not treat the result as a guaranteed deadline. Use the calculator to make uncertainty visible, compare scenarios, and support a schedule that includes explicit risk reserves and regular updates.