What does PERT stand for?

PERT stands for Program Evaluation and Review Technique. It was developed by the US Navy in the 1950s for the Polaris missile project and is now widely used in project management to handle estimation uncertainty.

Why does PERT weight the most likely estimate by 4?

The formula models task duration as a beta distribution. The most likely value is given four times the weight of the optimistic and pessimistic values because real-world task durations cluster around the mode. The result is a better approximation of the statistical mean than a simple average.

What does standard deviation tell me here?

Standard deviation (SD = (P - O) / 6) measures how spread out the possible durations are. A large SD means high uncertainty; a small SD means your three estimates are close together and the result is reliable. The 3-sigma range (Expected ± 3 × SD) should contain roughly 99.7% of real outcomes.

Can I mix units — hours for one task, days for another?

Each task row uses the unit you select for that row. The summary total only makes sense when all tasks share the same unit. If you mix units, read the per-row results individually rather than the sum.

Is the pessimistic value the absolute worst case?

It should represent a realistic bad-case scenario — not an impossible catastrophe. A common guideline is: the worst outcome you would expect to see if the task were repeated 100 times. Extreme outliers (force majeure, project cancellation) are usually handled separately in risk registers.

PERT Estimate Calculator

Name: PERT Estimate Calculator
Availability: InStock
Author: Nham Vu

Enter optimistic, most likely, and pessimistic durations to get a PERT weighted estimate, standard deviation, and confidence range.

Task Inputs

Enter three time estimates per task.

Default unit

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Summary

Enter optimistic, most likely, and pessimistic durations to get a PERT weighted estimate, standard deviation, and confidence range.

How it works

Enter the optimistic duration — the best-case time if everything goes perfectly.
Enter the most likely duration — your realistic day-to-day estimate.
Enter the pessimistic duration — the worst-case time if major problems occur.
Select the unit (hours, days, weeks, or months) for all three values.
The PERT expected duration is computed as (O + 4M + P) / 6.
Standard deviation is (P - O) / 6; the confidence range is Expected ± 3 × SD.

Use cases

Estimate software development task durations more accurately than single-point guesses.
Prepare project schedules with realistic buffer time baked in.
Compare multiple tasks side by side using the multi-task table.
Communicate uncertainty to stakeholders with a clear confidence range.
Use in agile sprint planning to size stories with three-point estimates.
Apply to construction, research, or any project with variable durations.
Feed the expected value and variance into critical path analysis.
Train team members on structured estimation techniques.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu