PERT Calculator (Three-Point Estimate)

How it works

E = (O + 4M + P) ÷ 6 · σ = (P − O) ÷ 6 · Triangular = (O + M + P) ÷ 3 · P(finish ≤ T) = Φ((T − E) ÷ σ) · For a chain of tasks: E_total = ΣE, σ_total = √(Σσ²)

PERT (Program Evaluation and Review Technique), developed for the US Navy Polaris program in 1958, models each estimate as a beta distribution that leans toward the most likely value. The weighted mean (O + 4M + P) ÷ 6 and the standard deviation (P − O) ÷ 6 feed a normal approximation that gives the chance of meeting a deadline. The PMI PMBOK Guide lists it as the three-point estimating technique; the simpler triangular average (O + M + P) ÷ 3 is shown for comparison.

Worked example

A task estimated at 4 days optimistic, 6 days most likely and 14 days pessimistic has E = (4 + 4 × 6 + 14) ÷ 6 = 7.00 days and σ = (14 − 4) ÷ 6 = 1.67 days. A 9-day target is (9 − 7) ÷ 1.67 = 1.2 standard deviations above the mean, so the chance of finishing within 9 days is 88.49%. The triangular estimate is 8.00 days, and the 95% range is 3.67 – 10.33 days.

Sources: Program evaluation and review technique — overview and formulas · Three-point estimation (PERT and triangular)

Frequently asked questions