PWPM Wiki

How to Calculate PERT Estimates

A practical, step-by-step guide to calculate pert estimates. Program Evaluation and Review Technique is the concept behind it — here is how to do it in practice.

For the full concept, see Program Evaluation and Review Technique.

Steps

  1. For each activity, gather three estimates: Optimistic (O), Most Likely (M) and Pessimistic (P).
  2. Compute the expected duration: TE = (O + 4M + P) ÷ 6.
  3. Compute the standard deviation: SD = (P − O) ÷ 6, and variance = SD².
  4. Use TE values to find the critical path.
  5. Sum the variances of critical activities, take the square root for the path’s SD, and use it to state completion confidence.