PERT Estimate Calculator
Three-point estimation: weighted average, standard deviation, and P80/P90 from optimistic, most likely and pessimistic values.
How the PERT estimate is calculated
The weighted average is (O + 4M + P) / 6 — the most likely estimate counts four times as much as either extreme, which approximates the mean of a beta distribution fitted to the three points. Standard deviation is (P − O) / 6, based on the beta distribution's typical range spanning roughly six standard deviations. P80 and P90 add a normal-approximation z-score multiple of that standard deviation on top of the mean, giving a single number to budget or schedule against at your desired confidence level.
Once you have PERT estimates for each activity in a project, they can feed directly into a Critical Path Method schedule as the duration inputs, replacing single fixed-duration guesses with a range-aware estimate. The spread between your P80/P90 figures and the mean estimate is also a useful starting point for sizing a contingency reserve.
Frequently asked questions
- Why weight the most likely estimate 4x?
- PERT (Program Evaluation and Review Technique) models the estimate as a beta distribution where the most likely value dominates but the optimistic and pessimistic bounds still pull the mean toward them — the (O + 4M + P) / 6 formula approximates that distribution's mean using only three points.
- What do P80 and P90 mean?
- The estimate you'd need to budget or schedule for to have an 80% or 90% chance of not being exceeded, assuming a normal approximation to the PERT distribution using its mean and standard deviation.
- Can I use this for cost estimates too, not just duration?
- Yes — PERT works identically whether O, M and P are durations, costs, or any other single quantity with a three-point estimate.
- What is PERT estimation?
- PERT (Program Evaluation and Review Technique) is a three-point estimating method that combines optimistic, most likely, and pessimistic estimates into a single weighted average and confidence range, rather than relying on one uncertain point estimate.
- Why use three-point estimation instead of a single estimate?
- A single-point estimate hides uncertainty and is frequently wrong in a specific direction — usually optimistic. Three-point estimation captures the range of plausible outcomes explicitly, producing both a more realistic expected value and a measure of how much uncertainty surrounds it.
- What does the standard deviation in PERT represent?
- It quantifies how much uncertainty exists in the estimate, based on the spread between optimistic and pessimistic values — a wider O-to-P range produces a larger standard deviation, which in turn widens the gap between the mean estimate and the P80/P90 confidence figures.
- How does PERT differ from CPM?
- The Critical Path Method uses a single fixed duration per activity to compute a deterministic schedule. PERT instead uses three-point estimates per activity to account for uncertainty — the two are often used together, with PERT estimates feeding into a CPM schedule as the duration inputs.