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Calcimator

PERT Estimate Calculator

Calculate weighted duration from optimistic, most likely, and pessimistic estimates.

Inputs

days
days
days

Results

Expected Duration (PERT)

10.8 days

Standard Deviation2.5 days
Variance6.3
Range (Low)5.8 days
Range (High)15.8 days
Total Range10 days
Risk Factor (CV)23.1%
How to Use This Calculator
  1. Enter the optimistic (O), most likely (M), and pessimistic (P) duration estimates for the task.
  2. The calculator computes the PERT weighted average: (O + 4M + P) / 6.
  3. Review the PERT estimate and the standard deviation (P - O) / 6.
  4. Use the standard deviation to calculate confidence intervals for schedule risk analysis.
  5. Sum PERT estimates across critical path tasks to get an overall project duration estimate.

How the result changes with Most Likely (M)

Most Likely (M)Expected Duration (PERT)
1,000670.8 days
3,5002,337.5 days
6,5004,337.5 days
9,0006,004.2 days

What each input means

Optimistic (O)
Best-case scenario duration.
Most Likely (M)
Most probable duration.
Pessimistic (P)
Worst-case scenario duration.
Confidence Level (1=68%, 2=95%, 3=99.7%)
Number of standard deviations for confidence interval.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Optimistic (O) = 5, Most Likely (M) = 10, Pessimistic (P) = 20, Confidence Level (1=68%, 2=95%, 3=99.7%) = 2 = 4 input(s) provided
  2. Calculate Expected Duration
    10.8 = 10.8
  3. Calculate Standard Deviation
    Standard Deviation
    2.5 = 2.5
  4. Calculate Variance
    Variance
    6.3 = 6.3

Engine last updated . Checked against 1 independently-derived test — how we verify calculators.

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