THE NUMBERS, EXPLAINED
How to use this calculator.
Uses the PERT weighted estimate. It is a planning approximation, not a guaranteed completion interval.
Start with your inputs
Enter optimistic duration, most likely duration, pessimistic duration. Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Optimistic duration | 2 |
| Most likely duration | 5 |
| Pessimistic duration | 8 |
A worked example
With the example inputs above, pert expected duration is 5. Change the inputs to explore your own scenario.
Putting the example numbers into the formula
PERT expected duration
((2)+4 × (5)+(8)) ÷ 6 = 5
Estimated standard deviation
((8)-(2)) ÷ 6 = 1
| Result | Example answer |
|---|---|
| PERT expected duration | 5 |
| Estimated standard deviation | 1 |
How this is calculated
PERT expected duration
(a+4 × b+c) ÷ 6
Estimated standard deviation
(c-a) ÷ 6
Symbols and units
- a
- Optimistic duration
- b
- Most likely duration
- c
- Pessimistic duration
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
Uses the PERT weighted estimate. It is a planning approximation, not a guaranteed completion interval.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
PERT expected duration; Estimated standard deviation. The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: Uses the PERT weighted estimate. It is a planning approximation, not a guaranteed completion interval.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
Read the quality report →