Project Timeline Estimator
Estimate total project duration and schedule risk from optimistic, likely, and pessimistic per-task time estimates (PERT).
Last reviewed
PERT project timelines turn optimistic, likely, and pessimistic estimates for each task into a statistically weighted duration. It's calculated as Expected Time = (Optimistic + 4×Most Likely + Pessimistic) / 6, summed across tasks with variance combined into a risk window.
PERT Estimate
30 Days
Peak Risk Duration
37 Days
Timeline Uncertainty
±3.2 Days
Tasks
Statistical Timeline
Weighted Load Analysis
How PERT Estimation Works
How to use this calculator
For each task, enter three duration guesses: the optimistic case (everything goes right), the most likely case, and the pessimistic case (real-world delays and blockers). PERT (Program Evaluation and Review Technique) combines the three into a single weighted expected duration per task, then combines every task's uncertainty into a project-wide risk window. It was developed by the U.S. Navy in the 1950s for the Polaris missile program specifically because task durations on a novel project are uncertain, not because it targets any single cognitive bias.
The formula
Expected Duration (per task)
The Most Likely estimate (M) gets 4x the weight of the two extremes, approximating a Beta distribution where outcomes cluster near the realistic middle case rather than the edges. Each task's spread, SD = (P − O) / 6, is combined across all tasks (variances add, so the project SD is the square root of the summed task variances) to produce the Timeline Uncertainty figure above.
A worked example
With the three seeded tasks - Discovery (3/5/9 days), Design (5/8/15 days), and Development (10/15/25 days) - each task's expected duration is 5.3, 8.7, and 15.8 days, summing to a 29.8-day PERT Estimate (shown rounded up as 30 Days). Each task's standard deviation is 1.0, 1.7, and 2.5 days; combining the variances gives a project-wide ±3.2-day Timeline Uncertainty, so the ~95% confidence ceiling lands around 36.2 days (rounded up to 37).
Assumptions and limitations
This calculator assumes tasks run sequentially with no dependencies modeled beyond simple addition, so it doesn't account for parallel work streams, resource contention, or a critical path that differs from the task list order. The confidence ceiling uses ±2 standard deviations as an approximation of a 95% interval (the more precise multiplier is 1.96), which is the same rounding convention most PERT references use. Your three-point estimates are only as good as the judgment behind them - garbage estimates in still produce a confident-looking but wrong number out.
Frequently Asked Questions
?What is PERT?
Project Evaluation and Review Technique (PERT) is a statistical tool used in project management to analyze the tasks involved in completing a project.
?Why the 4x weight on 'Likely'?
The PERT formula uses a weighted average (Beta Distribution) to account for the fact that the most likely scenario is statistically much more probable than the extremes.
?What does the Timeline Uncertainty (±) figure mean?
It's the project's standard deviation - a measure of how much the total duration could realistically swing above or below the PERT Estimate, based on the spread between each task's optimistic and pessimistic guesses. A larger ± figure means your project is more susceptible to schedule surprises.
About this calculator. Results are estimates for education and planning, based on the inputs you enter and the published formula described above. Everything runs in your browser; nothing you type is sent to us or stored. It is not financial, tax, legal or medical advice. Read the disclaimer and our methodology.
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