Task Time Estimate (PERT) Calculator
A single number is a poor task estimate because real work has uncertainty. The PERT three-point method turns an optimistic, a most likely, and a pessimistic estimate into a weighted expected time plus a measure of spread. This calculator computes the PERT expected duration, the standard deviation, and the one and two standard-deviation confidence ranges, so you can commit to a realistic window rather than a point estimate. Use any consistent time unit; the outputs share that unit.
PERT estimate formula
Expected time = (O + 4M + P) / 6
Standard deviation = (P - O) / 6
68% range = expected +/- 1 standard deviation
95% range = expected +/- 2 standard deviations
The most likely estimate carries four times the weight of the extremes, approximating a beta distribution. The spread assumes O to P covers about six standard deviations.
Three-point estimating context
- PERT was developed for the U.S. Navy Polaris program and is a standard schedule-risk method.
- Wider gaps between optimistic and pessimistic estimates produce a larger standard deviation.
- Quote the 68% or 95% range to stakeholders rather than a single point estimate.
- Use consistent units for all three inputs; outputs share the same unit.
- Summing expected times across tasks gives a project estimate; standard deviations combine in quadrature.
PERT estimating: frequently asked questions
What is the PERT estimate?
PERT (Program Evaluation and Review Technique) combines three estimates: optimistic (O), most likely (M), and pessimistic (P). The expected time is the weighted average (O + 4M + P) divided by 6, which gives the most likely value four times the weight of the extremes. This is the beta-distribution approximation used in project scheduling.
How is the standard deviation found?
The PERT standard deviation is (P - O) divided by 6. It assumes the spread between the optimistic and pessimistic estimates covers about six standard deviations. A wider gap between O and P means more uncertainty and a larger standard deviation.
What are the confidence ranges?
Adding and subtracting one standard deviation from the expected time gives roughly a 68% confidence interval; two standard deviations gives about 95%. This calculator reports the expected time plus and minus one and two standard deviations so you can quote a range rather than a single number.
Why weight the most likely estimate four times?
The (O + 4M + P) / 6 formula approximates the mean of a beta distribution skewed toward the most likely outcome. Giving M four times the weight reflects that the realistic estimate should dominate while the optimistic and pessimistic bounds still pull the mean toward whichever tail is longer.
Where does PERT come from?
PERT was developed for the U.S. Navy Polaris program and is documented in project management references including those published by the Project Management Institute. It is a standard part of schedule risk analysis used across government and industry programs.
Official sources
- Project Management Institute: project management standards.
- NASA: NASA Technical Reports Server: schedule estimation.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.