Three-Point Estimator
Estimate project duration using PERT three-point estimation technique
Range around the expected estimate at each confidence level, derived from the standard deviation.
Add a row per task. Totals use the same distribution method; variances add, so the project standard deviation is the square root of the summed variances.
| Task | Optimistic (O) | Most likely (M) | Pessimistic (P) | Expected |
|---|
What is Three-Point Estimator?
The Three-Point Estimator is a free tool that produces a more realistic project estimate using the PERT technique. Instead of committing to a single number, you give three estimates for a task: optimistic (best case), most likely (realistic), and pessimistic (worst case). The tool applies the PERT formula (O + 4M + P) / 6 for a weighted expected time, then computes the standard deviation and variance to show how much uncertainty surrounds the estimate. You can switch to a simple triangular average (O + M + P) / 3, pick a confidence level (68% to 99%) to see the matching range, apply quick presets, and add multiple task rows that roll up into one project estimate — all in your browser with no signup.
How to use Three-Point Estimator?
Turning three guesses into a reliable estimate takes only moments:
- 1 Enter your three time estimates: the optimistic value where everything goes right, the most likely value reflecting normal conditions, and the pessimistic value where problems pile up. Use the same unit for all three — or load a quick preset to start fast.
-
2
Choose a distribution: PERT (beta) weights the most likely estimate four times with
(O + 4M + P) / 6, while triangular treats all three equally with(O + M + P) / 3. The expected value updates instantly. -
3
Review the standard deviation, calculated as
(P - O) / 6, then pick a confidence level. The tool multiplies it by the matching z-score (1, 1.645, 1.96, or 2.576) to show a lower bound, an upper bound, and a commit-to value. - 4 Add a row per task in the project table to estimate a whole plan. Expected times sum directly, variances add, and the project standard deviation is the square root of that total.
Why use this tool?
Single-point estimates feel precise but are almost always wrong, because they hide the uncertainty every real task carries. By forcing you to think through the best, likely, and worst cases, three-point estimation produces a number that is both more honest and more defensible. The PERT weighting anchors the estimate to the realistic case while accounting for trouble, and the standard deviation quantifies risk so you can set sensible buffers. Confidence levels translate that risk into a concrete range, and the multi-task table rolls individual tasks into a trustworthy project total. Everything runs locally in your browser, so your estimates stay private. The technique is widely used in project management because it improves accuracy and helps teams set commitments stakeholders can trust.
Examples
For a feature you give optimistic 4 days, most likely 6, and pessimistic 14. PERT returns (4 + 24 + 14) / 6 = 7 days expected, well above the most likely value because of the long worst case. Triangular would instead give (4 + 6 + 14) / 3 = 8 days.
With expected 7 and standard deviation 2, the 95% range is 7 ± 1.96 × 2, roughly 3.1 to 10.9 days. Quote the upper bound when you need high confidence in the deadline.
Add three tasks expected at 5, 7, and 4 days for a total of 16. Their variances add, so the project standard deviation is the square root of the summed variances — tighter than adding worst cases.
Frequently Asked Questions
What is the PERT formula used here?
The expected time is calculated as (O + 4M + P) / 6, where O is optimistic, M is most likely, and P is pessimistic. The most likely value is weighted four times because it is the most probable outcome.
What is the difference between PERT and triangular?
PERT (beta) weights the most likely value four times with (O + 4M + P) / 6. Triangular uses a plain average (O + M + P) / 3, giving all three values equal weight. Pick triangular when you have little reason to trust the most likely value more than the extremes.
How are the confidence ranges calculated?
The tool multiplies the standard deviation by a z-score for the chosen level — 1 for 68%, 1.645 for 90%, 1.96 for 95%, and 2.576 for 99% — then adds and subtracts that margin from the expected estimate.
How does the multi-task project total work?
Each task is estimated independently, then expected times are summed. Variances add too, so the project standard deviation is the square root of the total variance — tighter than adding worst cases, which overstates combined risk.
How should I choose optimistic and pessimistic values?
Treat optimistic as a roughly 10 percent chance of finishing faster and pessimistic as a roughly 10 percent chance of running longer. They are realistic extremes, not the absolute fastest or slowest imaginable.
Is my estimate data saved anywhere?
No. All PERT calculations happen in your browser and nothing is uploaded, so your estimates and assumptions remain entirely private.
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