Task Time Estimate Calculator
Single-number estimates are usually wrong because they ignore uncertainty. The PERT three-point method asks for an optimistic, most likely, and pessimistic duration and combines them into a weighted expected estimate, giving the most likely value the greatest weight. This calculator returns the PERT expected duration, the standard deviation, and confidence ranges at one and two standard deviations so you can plan with the spread in mind rather than a single point.
PERT formula
E = (O + 4M + P) / 6
standard deviation = (P - O) / 6
68% range = E plus or minus 1 SD
95% range = E plus or minus 2 SD
O is the optimistic (best case) time, M the most likely time, and P the pessimistic (worst case) time. The expected estimate weights M four times. The standard deviation captures uncertainty as the spread between worst and best cases divided by six.
Worked example
For O = 2, M = 5, P = 14: E = (2 + 4 * 5 + 14) / 6 = 36 / 6 = 6.00. Standard deviation = (14 - 2) / 6 = 2.00. The 68% range is 4.00 to 8.00 and the 95% range is 2.00 to 10.00. So although the most likely time is 5, the expected estimate of 6 reflects the long tail toward 14.
Task estimates: frequently asked questions
What is the PERT three-point estimate?
PERT (Program Evaluation and Review Technique) estimates task duration from three numbers: an optimistic time (O), a most likely time (M), and a pessimistic time (P). The expected duration is the weighted average E = (O + 4M + P) / 6, which gives four times the weight to the most likely value. It produces a more realistic estimate than a single guess.
How is the standard deviation calculated?
PERT approximates the standard deviation of a task as (P - O) / 6, the spread between the pessimistic and optimistic times divided by six. A larger gap means more uncertainty. You can use it to build confidence ranges: about 68% of outcomes fall within one standard deviation of the expected estimate.
Why weight the most likely value four times?
PERT assumes task durations roughly follow a beta distribution, where the most likely outcome is far more probable than the extremes. Weighting it four times, against one each for the best and worst cases, reflects that the realistic estimate dominates while still accounting for both tails.
What confidence range does this give?
The calculator shows the expected estimate plus and minus one and two standard deviations. Roughly 68% of outcomes fall within one standard deviation and about 95% within two. These are planning ranges, not guarantees, and depend on the quality of your three input estimates.
Sources and method
- PERT was developed by the U.S. Navy Special Projects Office in the 1950s; the beta three-point estimate E = (O + 4M + P) / 6 is its standard formula.
- The expected value and standard deviation are fixed formulas computed directly by this tool, not estimated.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.