What is three-point estimation?

Three-point estimation is a technique that uses three values — optimistic (O), most likely (M), and pessimistic (P) — to calculate a weighted average duration. The PERT formula E = (O + 4M + P) / 6 gives more weight to the most likely value.

What is the difference between PERT and simple average?

A simple average treats all three values equally: (O + M + P) / 3. PERT weights the most likely value four times more heavily, producing a more realistic estimate when the distribution is skewed.

What does standard deviation tell me?

Standard deviation SD = (P - O) / 6 measures the uncertainty of your estimate. A large SD means high variance — the task could take anywhere in a wide range. Add 1 SD for ~68% confidence, or 2 SD for ~95% confidence.

What units should I use?

Any consistent unit works — hours, days, weeks, or story points. The calculator outputs results in the same unit you enter. Just make sure all three values for a task use the same unit.

Can I estimate a whole project?

Yes. Use the multi-task table to add all tasks. The tool sums the PERT estimates and combines standard deviations statistically (square root of the sum of squared SDs) to give a total project range.

Why must optimistic be less than or equal to pessimistic?

By definition, the optimistic value is the shortest duration (best case) and pessimistic is the longest (worst case). If O > P the estimate is logically inconsistent.

Three Point Estimate

Name: Three Point Estimate
Availability: InStock
Author: Nham Vu

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

Single Task Estimate

Task Name (optional)

Optimistic (O)

Most Likely (M)

Pessimistic (P)

Unit

Project Tasks

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Enter O, M, P values and click Calculate

Results will appear here

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Summary

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

How it works

Enter the optimistic duration — the best-case scenario if everything goes smoothly.
Enter the most likely duration — your realistic day-to-day expectation.
Enter the pessimistic duration — the worst case if major issues arise.
The tool computes the PERT weighted average: (O + 4M + P) / 6.
Standard deviation (P - O) / 6 is used to calculate confidence interval ranges.
Add multiple tasks to sum up total project estimates with combined uncertainty.

Use cases

Estimate software development sprints with realistic buffers.
Plan construction or manufacturing project timelines.
Produce risk-adjusted schedules for project proposals.
Identify high-variance tasks that need extra risk mitigation.
Compare optimistic vs. pessimistic scenarios for stakeholder presentations.
Aggregate multi-task estimates into a total project duration range.
Support agile story-point planning with time-based equivalents.
Teach project management estimation concepts in training workshops.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu