How to Calculate Time Estimates Using PERT

PERT considers the best and worst case scenarios for project length.
PERT considers the best and worst case scenarios for project length. (Image: Mike Watson Images/moodboard/Getty Images)

Part of project management is figuring how long the project will take. PERT -- the Program Evaluation and Review Technique -- is a tool for estimating when you'll be done. You start by estimating the best, worst and most likely time frame for the project. You then use the PERT formula to add the estimates together.

The Three Estimates

First, estimate the least amount of time the project would take if everything went well. Then take the pessimistic view by estimating how long it will take to finish if everything goes haywire. The third estimate is the realistic one: how long you think your project will probably take. These three times are the basis of the PERT calculation.

Calculating PERT

The calculation is simple. Suppose your optimistic scenario is three weeks, your pessimistic estimate is eight weeks and your most likely time frame is four weeks. Multiply the "most likely" figure by four to arrive at 16. To this figure, add the other two figures -- 3 and 8 -- to arrive at a total of 27 weeks. Divide by six to get your PERT best estimate. In this example you end up with 4.5 weeks as the estimated project duration. The key to the PERT calculation is that the most likely estimate carries four times the weight as the other two estimates.

When to Use PERT

If you're confident the "most likely" estimate is accurate, it's reasonable to stick with that figure and not worry about calculating PERT for your project. PERT is more useful when you have less confidence in the outcome, or when there's a big gap between the optimistic and pessimistic time frames.

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