Free PM calculator · Estimating
PERT Three-Point Estimate Calculator
Enter optimistic, most likely and pessimistic estimates for a task, in days, hours or dollars. You get the PERT (beta) estimate, the triangular average, the standard deviation and confidence ranges.
Enter values where optimistic ≤ most likely ≤ pessimistic.
}What is a PERT three-point estimate?
PERT (Program Evaluation and Review Technique) handles uncertainty by asking for three estimates instead of one. The weighted average gives the most likely value four times the weight of the extremes. The standard deviation shows how uncertain the estimate is.
Triangular mean = (O + M + P) ÷ 3
Standard deviation σ = (P − O) ÷ 6 · Variance = σ²
Worked example
A data migration could take 4 days if all goes well, most likely 6 days, and 14 days in the worst case. PERT = (4 + 24 + 14) ÷ 6 = 7 days. The triangular average is 8 days. σ = (14 − 4) ÷ 6 ≈ 1.67 days, so there is about a 95% chance of finishing between 3.7 and 10.3 days.
Combining several tasks
For tasks in sequence, add their PERT means to get the total. To get the total standard deviation, add the variances (not the standard deviations) and take the square root. For a full picture of many uncertain tasks, use the Monte Carlo schedule simulator. For a network of tasks with dependencies, use the PERT/CPM critical path tool.
Related terms: PERT, three-point estimate, standard deviation.