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PERT / Three-Point Estimation Calculator

Free tool
5.5

expected value

(O + 4M + P) / 6 - confidence range 4.3 to 6.7 (±1.2)

  • The spread between best and worst case is wide relative to the estimate - that usually means the work isn't well understood yet, not that the math is wrong.
  • This is a confidence interval for a schedule, not a group consensus - it works well for one epic, not for sprint-level story sizing.

Sizing sprint-ready stories instead? Vote them as a team.

What the three inputs actually mean

Three-point (PERT) estimation asks for three numbers instead of one: optimistic (best case, everything goes right), most likely (what you'd honestly expect), and pessimistic (worst case, things go wrong). Combining them into a single expected value - weighted toward the most-likely case - produces a defensible number along with an implicit confidence range, instead of one guess presented as certainty.

The formula

Expected value = (O + 4M + P) / 6

The most-likely estimate counts four times as much as either extreme, which is why the result usually lands close to your "normal" case rather than splitting the difference between best and worst. The spread between optimistic and pessimistic also gives you a standard deviation - (P − O) / 6 - which is what the range shown above the calculator comes from.

When this beats planning poker

Three-point estimation and planning poker solve a similar problem differently. Poker produces a single relative estimate through team consensus and discussion - it's built for sprint-ready stories, where the conversation surfaces hidden complexity. Three-point produces a probability-weighted number with an explicit confidence interval, which fits better when a stakeholder needs a schedule for one epic or feature, not a story-by-story sprint plan. Full comparison: three-point estimation vs. planning poker.

What a wide spread is telling you

If pessimistic is far from optimistic, the math still works - but a spread that's wide relative to the estimate itself is usually a signal that the work isn't well understood yet, not a math problem. That's worth a scoping conversation before the number goes into a schedule anyone commits to.

Frequently asked questions

Expected value = (optimistic + 4 × most likely + pessimistic) / 6. The most-likely case counts four times as much as either extreme, so the result leans toward what you honestly expect rather than splitting the difference.
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