PERT Three-Point Estimator
Enter optimistic, most likely and pessimistic estimates for each task. Get the expected total, standard deviation, 68/95/99.7% ranges and the probability of hitting your target.
| Task | Optimistic | Most likely | Pessimistic | Expected | σ |
|---|
Why three-point estimates beat single numbers
A single estimate hides uncertainty. Asking for three numbers forces the estimator to think about what could go right and wrong, and gives you a range you can actually plan contingency around. PERT (Program Evaluation and Review Technique) was developed in the late 1950s for large defence programmes and is still the standard approach taught in project management courses.
How to collect good estimates
- Ask the person who will do the work, not the person who wants it done.
- Get the pessimistic number first — it surfaces risks early.
- For tasks with a pessimistic value more than 3× the optimistic, break the task down further.
- Record the assumptions behind each estimate in your risk register.
Reading the probability
The probability of finishing within the target uses the normal approximation of the summed task distributions. As a rule of thumb, commit externally to the 85–90% figure and manage internally to the expected value. Feed the expected durations into the critical path calculator to see which tasks drive the end date.
Frequently asked questions
What is the PERT formula?
Expected duration E = (O + 4M + P) ÷ 6, where O is optimistic, M is most likely and P is pessimistic. Standard deviation σ = (P − O) ÷ 6.
What is the difference between PERT and triangular estimates?
Triangular estimating uses (O + M + P) ÷ 3 and gives equal weight to each point. PERT (beta) weights the most likely value four times, which usually gives a more realistic central estimate.
Can I use story points or costs instead of days?
Yes. The maths works for any unit — days, hours, story points or money — as long as all rows use the same unit.