PERT Three-Point Estimate Calculator
Expected duration from optimistic, likely and pessimistic estimates, with confidence ranges.
Results
Expected duration
8.000
Standard deviation (sample)
2.000
Lower estimate
6.000
Upper estimate
10.000
Upper estimate
12.000
What this tool does
Asking for one number invites wishful thinking; asking for three forces you to name what could go wrong. The spread between best and worst case becomes the standard deviation, and the ranges show where roughly two thirds and ninety-five percent of outcomes should land.
Formula
E = (O + 4M + P) ÷ 6 ; σ = (P − O) ÷ 6
Variables
| Symbol | Meaning | Unit |
|---|---|---|
o | Optimistic estimate | — |
ml | Most likely estimate | — |
p | Pessimistic estimate | — |
E | Expected duration | — |
SD | Standard deviation (sample) | — |
C1 | Lower estimate | — |
C2 | Upper estimate | — |
C9 | Upper estimate | — |
Worked example
- Optimistic estimate4
- Most likely estimate7
- Pessimistic estimate16
- Expected duration8.000
- Standard deviation (sample)2.000
- Lower estimate6.000
- Upper estimate10.000
- Upper estimate12.000
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
Frequently asked questions
Why is the most likely value weighted four times?
Because the realistic case deserves more weight than the two extremes, which are by definition unlikely. The 1-4-1 weighting comes from approximating a beta distribution and is the convention PERT has used since the 1950s. The effect is that a long tail of possible delay pulls the estimate up without dominating it.