Free calculator
Three-point estimate calculator (PERT and triangular)
Enter an optimistic, a most likely and a pessimistic value for a duration or a cost. The calculator returns the PERT and triangular estimates, the standard deviation and an approximate 80% confidence value.
Calculator
The formulas
| E = (O + 4M + P) / 6 | PERT (beta) estimate: weights the most likely value four times |
|---|---|
| E = (O + M + P) / 3 | triangular estimate: all three values count equally |
| σ = (P − O) / 6 | standard deviation of the PERT estimate |
| P80 ≈ E + 0.84 σ | the value met in about 80% of cases, assuming a normal distribution |
Worked example
An activity should take 14 days. It could take 10 if everything goes well and 26 if approvals are slow.
The PERT estimate is (10 + 4 × 14 + 26) / 6 = 15.33 days, and the triangular estimate is (10 + 14 + 26) / 3 = 16.67 days. The standard deviation is (26 − 10) / 6 = 2.67 days, so an 80% confidence duration is about 15.33 + 0.84 × 2.67 = 17.57 days. Plan with 14 and you are more likely than not to be late.
For a whole schedule, single-activity estimates are not enough, because paths merge and risks happen. That is what a Monte Carlo schedule risk analysis is for.
Common questions
PERT or triangular?
Use PERT when you trust the most likely value. Use triangular when you do not, because it gives the optimistic and pessimistic ends more weight.
Bring one schedule. See your critical path in an hour.
Free during our launch until 31 March 2027.
