Three-point estimation using PERT (beta) or triangular distribution. Enter optimistic, most likely, and pessimistic durations to calculate expected duration, standard deviation, and confidence ranges.
tE = (O + 4M + P) / 6
Gives 4x weight to the most likely estimate. This is the formula PMI uses on the PMP exam unless stated otherwise. The “4” reflects the beta distribution assumption.
tE = (O + M + P) / 3
Gives equal weight to all three estimates. Use when there is no reason to weight the most likely value more heavily. Less common on the PMP exam but still fair game.
σ = (P - O) / 6
Measures the spread of uncertainty. A wider gap between optimistic and pessimistic means more risk. This formula is the same for both PERT and triangular methods.
σ² = [(P - O) / 6]²
The square of the standard deviation. Critical for multi-activity aggregation: you sum variances across activities, then take the square root for the combined standard deviation.
Total Expected = Σ tE
Total Variance = Σ σ²
Combined σ = √(Total Variance)
When estimating a project with multiple activities on the critical path, sum the expected durations and sum the variances (not the standard deviations). Then take the square root of the total variance to get the project-level standard deviation. This assumes activities are independent.
Want to practice these formulas under exam conditions? Try 10 free PMP practice questions or explore more PMP study resources.
Program Evaluation and Review Technique (PERT) is a three-point estimation method that calculates an expected duration from optimistic (O), most likely (M), and pessimistic (P) values to account for uncertainty.
Expected duration = (O + 4M + P) / 6. Standard deviation = (P − O) / 6. The 4× weighting on the most likely value produces a beta distribution.
Triangular estimation uses (O + M + P) / 3, weighting all three points equally. PERT (beta) weights M four times more heavily, which produces a tighter, more realistic forecast when the most likely value is well understood.
Use PERT when uncertainty is high and a single estimate would be misleading. The exam expects you to recognize PERT as the more rigorous of the two three-point techniques and to calculate expected duration and standard deviation cleanly.
PERT and other estimation techniques sit in the Process domain (41% of the 2026 exam). Expect calculation questions and concept questions on when to use PERT vs analogous, parametric, or triangular estimates.
