PERT Estimate Calculator
Make three-point PERT estimates for tasks and see the chance of meeting a deadline.
About the PERT Estimate Calculator
List each task with optimistic, most likely and pessimistic estimates, and an optional deadline. The calculator gives the expected total, its standard deviation, a 95% range and the chance of meeting the deadline.
PERT (Program Evaluation and Review Technique) asks for an optimistic, most likely and pessimistic estimate and weights the most likely four times. For tasks done one after another, expected times add and variances add, so the total's standard deviation is the square root of the summed variances. The deadline probability assumes the total is roughly normal.
The formula
E = (O + 4M + P) ÷ 6; σ = (P − O) ÷ 6; σ_total = √Σσ²
Worked example
Design (3, 5, 10), build (8, 12, 20) and test (2, 4, 9) days
- Expected: 5.5 + 12.67 + 4.5 = 22.67 days
- Standard deviation 2.59 days
- About 70% likely to finish within 24 days
Questions people ask
Do the units matter?
Use any unit (hours, days) consistently.
What about parallel tasks?
Only the critical path (longest chain) sets the duration; enter those tasks.