Project Timeline Estimator
Find expected project duration from three estimates.
How the PERT three-point estimate works
The PERT (Program Evaluation and Review Technique) three-point estimate combines an optimistic, most-likely and pessimistic duration into a single expected duration and an uncertainty range — a standard project-management estimation method that's more realistic than a single best-guess number.
Enter your optimistic (best-case), most-likely, and pessimistic (worst-case) duration estimates, in the same unit (days is typical). The calculator returns the weighted expected duration and its standard deviation.
The most-likely estimate is weighted four times as heavily as the optimistic or pessimistic ones, reflecting the assumption that outcomes cluster around the realistic case more than the extremes.
The formula
Expected = (Optimistic + 4 × Most likely + Pessimistic) ÷ 6. SD = (Pessimistic − Optimistic) ÷ 6.
Weight the most-likely estimate four times, add the optimistic and pessimistic estimates once each, and divide by 6 for the expected duration. The standard deviation approximates the spread between the optimistic and pessimistic cases.
Worked example
Optimistic 5 days, most likely 8 days, pessimistic 17 days
Expected = (5 + 4×8 + 17) ÷ 6 = 54 ÷ 6 = 9 days. Standard deviation = (17 − 5) ÷ 6 ≈ 2 days.
Frequently asked questions
Why weight the most-likely estimate more heavily?
PERT assumes durations roughly follow a beta distribution clustered around the realistic case, with the optimistic and pessimistic cases as less likely extremes — the 4x weighting approximates that shape.
What can I do with the standard deviation?
It gives a sense of estimate uncertainty — a wide gap between optimistic and pessimistic (a large standard deviation) signals higher risk and less confidence in the single expected number.
Do the three estimates need to be in days?
No — use any consistent time unit (hours, days, weeks). The result will be in that same unit.
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