Project Duration Estimate Calculator
Calculate Project Duration Estimate step by step: PERT: E = (O + 4M + P) ÷ 6, σ = (P − O) ÷ 6.
In short
Formula: PERT: E = (O + 4M + P) ÷ 6, σ = (P − O) ÷ 6. Triangular: E = (O + M + P) ÷ 3.
What this calculator does
Calculate Project Duration Estimate step by step: PERT: E = (O + 4M + P) ÷ 6, σ = (P − O) ÷ 6. Worked example, questions and limitations included.
Use it to turn Optimistic duration (O), Most likely duration (M), Pessimistic duration (P) into a checked result you can compare, copy, or rerun with different assumptions.
The page shows the formula, a numeric worked example, and the assumptions that affect this project estimation calculation.
Inputs and what they mean
- Optimistic duration (O)
- — number value.
- Most likely duration (M)
- — number value.
- Pessimistic duration (P)
- — number value.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
Three-point estimating is described in the PMBOK Guide; the beta-PERT weights come from the original 1950s PERT method.
The ±1σ range assumes an approximately normal spread, which is a simplification.
For a chain of tasks, add the means and add the variances (σ²), not the standard deviations.
PERT: E = (O + 4M + P) ÷ 6, σ = (P − O) ÷ 6. Triangular: E = (O + M + P) ÷ 3.
Inputs used: Optimistic duration (O), Most likely duration (M), Pessimistic duration (P).
Worked example
O 10, M 15, P 26 days
- E = (10 + 60 + 26) ÷ 6 = 16 days.
- σ = 16 ÷ 6 = 2.67 days.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- Relies on the quality of the three estimates.
- Ignores dependencies and resource limits.
- Normal approximation is rough for a single task.
- Results are planning estimates built only from the figures you enter. They are not a guarantee of cost, schedule or delivery, and they do not replace your organisation's own project controls.
- The result depends on the values you enter for this project duration estimate calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Common questions
PERT or triangular?
PERT weights the most likely value more heavily; triangular treats all three equally and is more conservative when the tail is long. Check the formula, example, and limitations on this page before using the result for a real project estimation decision.
Can I add the ranges of several tasks?
Add means and variances, then take the square root of the total variance. Adding standard deviations overstates uncertainty.
Is the 68% range a promise?
No. It depends on the estimates being honest and independent; in practice ranges are often too narrow.
How do I use the Project Duration Estimate Calculator?
Enter the required values for Optimistic duration (O), Most likely duration (M), Pessimistic duration (P). The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Project Duration Estimate Calculator use?
PERT: E = (O + 4M + P) ÷ 6, σ = (P − O) ÷ 6. Triangular: E = (O + M + P) ÷ 3. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the Project Duration Estimate Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
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