The pert calculator works out probability of completion, standard deviation (sd) and pert estimate from desired completion time, most probable, optimistic and pessimistic. Enter what you have and the relationship is applied straight away, so you get the answer without working through the algebra yourself.
What Is the PERT Calculator?
The pert calculator turns desired completion time, most probable, optimistic and pessimistic into probability of completion, standard deviation (sd) and pert estimate.
It is a quick way to get from the numbers you have to the number you need, without setting up a spreadsheet or reaching for a formula.
How the PERT Works
Each result follows from the ones before it, and the calculator shows every step, so a surprising answer can be traced back to the input that caused it.
Round your inputs to the precision you actually know, and keep more precision than that in the intermediate steps.
Getting a Result You Can Trust
Run the calculation twice with slightly different inputs. A sensible calculator moves its output in proportion; a wild change points to a unit or a decimal point problem in what you entered.
Write the inputs down next to the answer. It takes a moment and it is the difference between a number you can explain and one you cannot.
A Worked Example
Working through an example, with desired completion time set to 10, most probable set to 5, optimistic set to 2, the calculator returns pert estimate of 20.33, and standard deviation (sd) of 16.33, and z score of -0.63265. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out probability of completion?
Enter desired completion time, most probable, optimistic and pessimistic. The calculator then works through the relationship and reports probability of completion, standard deviation (sd) and pert estimate.
What does the pert actually calculate?
It takes desired completion time, most probable, optimistic and pessimistic and converts them into probability of completion, standard deviation (sd) and pert estimate using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter desired completion time?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.
Is the answer exact?
It is as exact as the formula and the inputs allow. The method itself is the standard one for this problem, so the only sources of error are the figures you enter and the precision you ask for.