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Story Estimation Confidence Calculator

Work out story estimation confidence instantly with clear inputs, formula shown and shareable results.

About Story Estimation Confidence Calculator

PERT combines three-point estimates as (optimistic + 4 x likely + pessimistic) / 6, with standard deviation (pessimistic - optimistic) / 6. The value is that it produces both a central estimate and an uncertainty range, so commitments can be made at a stated confidence level rather than on a single number that is optimistic by construction.

Formula

PERT estimate

expected = (O + 4M + P) / 6; sd = (P - O) / 6; 95th percentile = expected + 1.645 x sd

Worked example

With the default values below, Story Estimation Confidence Calculator returns PERT expected estimate of 9.67. Change any input above and every figure here is recalculated in your browser.

Inputs used

Optimistic estimateminimum 0.1 points
5 points
Most likely estimateminimum 0.1 points
8 points
Pessimistic estimateminimum 0.1 points
21 points
Team velocityminimum 0.1 points
30 points

Results

PERT expected estimate
9.67 points
Estimate standard deviation
2.67 points
95th percentile estimate
14.05 points
Sprints at the p95 estimate
0.47 sprints

Step by step

  1. 1Values used
  2. 2PERT estimate
  3. 3PERT expected estimate= 9.67 points
  4. 4Estimate standard deviation= 2.67 points
  5. 595th percentile estimate= 14.05 points
  6. 6Sprints at the p95 estimate= 0.47 sprints

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 2 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 is the most likely value weighted four times?

PERT approximates a beta distribution where the mode carries most of the information while the extremes bound it. The weights make the mean tractable to compute.

Should a team commit to the expected value?

No. The expected value is met roughly half the time. Commit at the 80th or 95th percentile and treat the difference as schedule buffer.

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