PERT Estimate Calculator
A three-point (PERT) estimate for a task from its optimistic, most likely and pessimistic durations — the expected time, its standard deviation, and how long to allow for 80%, 90% and 95% confidence.
PERT weights the most likely estimate four times as heavily as the extremes: E = (O + 4M + P) ÷ 6.
How the pert estimate calculator works
PERT weights the most likely estimate four times as heavily as the extremes: E = (O + 4M + P) ÷ 6. The spread (P − O) ÷ 6 is the standard deviation, and because a sum of many such tasks is roughly normal, adding 0.84, 1.28 or 1.64 standard deviations gives the duration you can quote with 80%, 90% or 95% confidence. Use the same unit (hours or days) for all three inputs.
Formula: E = (O + 4M + P) ÷ 6; σ = (P − O) ÷ 6; allow E + z·σ
Worked examples
| Inputs | Expected duration | Note |
|---|---|---|
| 3 / 5 / 12 days | 5.83 days | 5.83 days, σ 1.5 |
| 2 / 4 / 6 hours | 4 hours | 4 hours |
| A wide range: 1 / 2 / 10 weeks | 3.17 weeks | 3.17 weeks, σ 1.5 |
FAQFrequently asked questions
Why weight the most likely estimate by four?
It approximates a beta distribution skewed toward the mode, which fits how tasks behave: most finish near the likely time, a few run badly over, almost none finish far early. The 4 gives the mean of that shape.
What does the standard deviation tell me?
How uncertain the task is. A 3/5/12 estimate has a σ of 1.5 days: about two-thirds of the time the task lands within 1.5 days of the expected 5.8. Wide O-to-P ranges mean you do not yet know enough to plan tightly.
Can I add the estimates for a project?
Add the expected values, and add the variances (σ²) then take the square root for the project σ — never add the standard deviations. The confidence bands then apply to the total.
How honest are my three numbers?
The pessimistic figure is the one people understate. Ask what happens if a dependency slips, a key person is away, or a review round is needed; “worst case” should be genuinely bad, not slightly late.
Where these figures come from
- ISO 8601 — Date and time — Representations for information interchange — the yyyy-mm-dd format and the week-numbering rules used here
- US Naval Observatory — Calendars — the Gregorian leap-year rule
- GOV.UK — bank holidays — bank holidays by nation (not applied here)
Last checked: September 2026. The Gregorian calendar rules do not change; working-day counts exclude Saturdays and Sundays only.