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PERT Estimate Calculator

Work out pert estimate instantly with clear inputs, formula shown and shareable results.

About PERT Estimate Calculator

PERT weights the most likely estimate four times against the optimistic and pessimistic extremes, producing a beta-distributed expected value. The standard deviation is the range divided by six, which lets you quote a confidence interval rather than a single misleading number.

Formulas

PERT expected value

Expected = (Optimistic + 4 x Most likely + Pessimistic) / 6

Standard deviation

SD = (Pessimistic - Optimistic) / 6

Worked example

With the default values below, PERT Estimate Calculator returns PERT expected duration of 21.67. Change any input above and every figure here is recalculated in your browser.

Inputs used

Optimistic estimate
12 days
Most likely estimate
20 days
Pessimistic estimate
38 days
Confidence level for the range
95

Results

PERT expected duration
21.67
Standard deviation
4.333
Upper bound at the confidence level
30.16
Lower bound at the confidence level
13.17

Step by step

  1. 1Values used
  2. 2PERT expected value
  3. 3Standard deviation
  4. 4PERT expected duration= 21.67
  5. 5Standard deviation= 4.333
  6. 6Upper bound at the confidence level= 30.16
  7. 7Lower bound at the confidence level= 13.17

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why weight the most likely estimate four times?

Because the beta distribution PERT assumes is unimodal with most probability mass around the mode. The 1-4-1 weighting approximates its mean.

Why divide the range by six?

Because a normal distribution spans roughly six standard deviations, so the optimistic-to-pessimistic range is treated as that full span.

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PERT Estimate Calculator – Calculate PERT expected duration