Wolf Office ToolsPERT Estimate Calculator
Three-point estimation for project managers and PMP practitioners.
PERT (Program Evaluation and Review Technique) provides a statistically-grounded method for estimating task durations when uncertainty exists. Enter your optimistic, most likely, and pessimistic estimates to calculate the expected duration, standard deviation, and confidence ranges.
Multi-Task Project Estimator
Add multiple tasks. The total PERT estimate and combined standard deviation are calculated automatically.
The PERT Formula Explained
σ = (P − O) / 6
Variance = σ²
The factor of 4 applied to the Most Likely estimate reflects the beta distribution assumption — real-world tasks tend to cluster around the most realistic estimate, not spread evenly between extremes.
FAQ
When should I use PERT vs simple average?
Always use PERT when there is meaningful uncertainty in your estimate. A simple average treats all three estimates equally, which under-weights the most likely scenario. PERT is the standard method in PMP, PMI, and ITIL frameworks.
How do I combine multiple PERT estimates?
Sum the individual Expected values for total expected duration. Combine standard deviations using the root-sum-of-squares formula: σ_total = √(σ₁² + σ₂² + … + σₙ²). This tool calculates both for you in the multi-task estimator.
What is a good σ/E ratio?
A lower ratio indicates more certainty. If σ/E > 0.3, your estimate range is very wide and you should investigate what is driving the pessimistic scenario.