PERT Calculator — Project Duration Estimator
PERT (Program Evaluation and Review Technique) turns three gut-feel estimates — best case, most likely and worst case — into a statistically grounded expected duration with confidence intervals. Enter your three estimates and get the weighted expected time, standard deviation and probability bands.
Time unit
Weighted average: (O + 4M + P) ÷ 6
- 1
Four-times weight on most likely
4 × 7 = 28 - 2
Weighted sum (O + 4M + P)
3 + 28 + 15 = 46 - 3
Expected duration (÷ 6)
46 ÷ 6 = 7,67The most-likely estimate is weighted four times more than either extreme.
Hvordan fungerer denne lommeregner?
PERT expected duration = (O + 4M + P) ÷ 6, where O is optimistic, M is most-likely and P is pessimistic. Standard deviation = (P − O) ÷ 6. The 95% confidence interval runs from E − 2σ to E + 2σ, giving a realistic range for planning.
Formel
How this is calculated
PERT was developed by the US Navy in the 1950s to manage the Polaris submarine program under high uncertainty. Rather than a single point estimate, it collects three: the optimistic time O (if everything goes right), the most likely time M (under normal conditions) and the pessimistic time P (if significant problems occur). The formula E = (O + 4M + P) ÷ 6 is a weighted average that gives four times as much weight to the most-likely estimate as to either extreme, producing a Beta-distribution-inspired mean.
The standard deviation σ = (P − O) ÷ 6 measures how uncertain the estimate is: a large gap between optimistic and pessimistic means high uncertainty and a wide bell curve. By convention the three-sigma rule maps the entire O-to-P range to ±3σ around E. This lets you read off confidence intervals: the 68% interval is E ± σ, the 95% interval is E ± 2σ.
PERT assumes the task duration follows a Beta distribution, and that the three-sigma assumption holds. In practice the formula is an approximation — real project durations are often positively skewed (delays are more common than gains). Use PERT confidence bands as planning guides, not guarantees; combine them with schedule buffers or Monte Carlo simulation for high-stakes projects.
Ofte stillede spørgsmål
The formula approximates the mean of a Beta distribution. The four-times weight on M reflects the practical observation that the most-likely outcome is far more probable than either extreme. It also makes the formula easy to compute without fitting a full Beta distribution.
CPM (Critical Path Method) uses a single deterministic duration per task, while PERT uses three estimates and treats durations as random variables. PERT is better for novel or uncertain tasks where the duration is genuinely unknown; CPM suits well-understood, repeatable work.
Yes. Run PERT on each task on the critical path, then sum the expected durations for the project total. The project variance is the sum of individual variances (if tasks are independent), and the project standard deviation is the square root of that sum.
Også kendt som
TG we-Calculate Editorial Team. (2026). PERT Calculator — Project Duration Estimator [Online calculator]. TG we-Calculate. https://we-calculate.com/da/calculator/pert-calculator
TG we-Calculate Editorial Team. "PERT Calculator — Project Duration Estimator." TG we-Calculate. 2026. https://we-calculate.com/da/calculator/pert-calculator.
TG we-Calculate Editorial Team, "PERT Calculator — Project Duration Estimator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/da/calculator/pert-calculator
@misc{wecalculate_pert_calculator, title = {PERT Calculator — Project Duration Estimator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/da/calculator/pert-calculator}}, year = {2026}, note = {TG we-Calculate} }
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