Schedule · tool 3 of 7

PERT · Three-point Estimation

PERT · Three-point Estimation: produces realistic estimates when there’s uncertainty, and turns one fragile date into a range.

Best for: Uncertain or new work · Time: Minutes per task
Domain: Schedule · Focus area: Planning

How it works

For each task you ask for three estimates: Optimistic (O), Most likely (M) and Pessimistic (P). The weighted average gives a realistic expected duration, and the spread tells you how uncertain it is.

• M counts four times, so one extreme guess doesn’t dominate.
• σ (standard deviation) measures uncertainty: the bigger, the riskier.
• Expected ± σ covers about 68% of outcomes; + 2σ about 95%.

How to apply it

1. Ask the person who will do the work for O, M and P — not the manager.
2. Compute the expected value and σ (try it below).
3. Use the expected value in the schedule; add buffer on the critical path based on the σ values.
4. Communicate ranges: “10 to 14 days, most likely 12” rather than a single promise.

Try it

Last reviewed 2026-11-02 against the PMBOK® Guide, 8th edition · Practitioner review pending · How this guide is made