PERT Calculator
Compute PERT expected time, standard deviation, and variance from optimistic, most likely, and pessimistic project estimates.
What Is PERT?
PERT (Program Evaluation and Review Technique) estimates task duration from three values: optimistic, most likely, and pessimistic. It is common in project planning when uncertainty is high and you need both an expected time and a spread.
PERT Formulas
$$E = \frac{O + 4M + P}{6}$$ $$\sigma = \frac{P - O}{6}$$ $$\text{Var} = \sigma^2$$$E$ is expected duration, $\sigma$ is standard deviation, and $O$, $M$, and $P$ are the optimistic, most likely, and pessimistic estimates. The most likely value receives four times the weight of each extreme.
How to Use the Results
Expected time is your planning baseline. Standard deviation shows uncertainty: a larger spread means the task outcome is less predictable. Combine PERT tasks carefully in schedules because dependencies and resource limits are not modeled here.
Related tools: Recruitment Process Duration Calculator and Dating Theory Calculator.
Frequently Asked Questions
What units should I use?
Use the same unit for all three estimates: days, hours, or weeks. The calculator preserves whatever unit you enter.
Must pessimistic be greater than optimistic?
Yes for meaningful spread. If $P = O$, standard deviation becomes zero even though real tasks still have uncertainty.
Why is the most likely estimate weighted four times?
PERT assumes the beta distribution peak sits near the most likely value, so it influences the expected duration more than either extreme.
Is PERT the same as a simple average?
No. A plain average of $O$, $M$, and $P$ treats all three equally. PERT shifts the result toward the most likely case.
Can I build a full project schedule here?
This tool handles one task at a time. Full schedules also need dependencies, resource calendars, and critical path analysis.
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