I've set a time objective for my project. 10 days, for example. The project took 15 days to complete.

I have to report the performance regarding the time of the project, as a %. My question is: how do I calculate it?

PS: me and my collegues have a dispute betwen 2 options:
a. 10/15
b. 100%+((10-15)/10)

  • You overran by 50%, or to put it another way, the schedule was 150% of planned duration.
    – Marv Mills
    Commented May 9, 2015 at 8:00
  • 2
    which makes you look better, a small or big number?
    – Ewan
    Commented May 11, 2015 at 10:12

5 Answers 5


Wiki have good example about project overrun (it is about cost overrun, but in our case it doesn't matter). The text below I took from this article and adapted for your question's example:

Time overrun can be described in multiple ways.

  • As a percentage of the total time expenditure
  • As a total percentage including and above the planned time
  • As a percentage of the time overruns to planned time

So, for your example these values will be:

  • The time overruns constituted 33% of the total time:

((ActualTime - PlannedTime)/ActualTime) * 100%

  • The time for your project increased to 150%:

(ActualTime/PlannedTime) * 100%

  • The time overruns exceeded the planned time by 50%:

((ActualTime - PlannedTime)/PlannedTime) * 100%

The final example is the most commonly used as it specifically describes the time overruns exclusively whereas the other two describe the overrun as an aspect of the total expense. In any case care should be taken to accurately describe what is meant by the chosen percentage so as to avoid ambiguity.


Actual time - planned time = variance at completion.

The OP's option (a) is the SPIt in Earned Schedule. In this example, it results in 0.6667, which is this project's performance index, a valuable metric for future planning.


It depends how you want to report on it. I suggest that each of the following are equally valid, and it is down to how you want to report it.

As a percentage over-run: (Actual time - Planned time) * 100 / Planned time, so you suffered a (15 - 10) * 100 / 10 = 50% over-run. I would use this for reporting externally.

As a time comparison: Actual time * 100 / Planned time, so you took 15 * 100 / 10 = 150% of the planned duration. I would use this as a learning point for future project estimates.

As a comparative development rate: Planned time * 100 / Actual time, so you achieved 10 * 100 / 15 = 66.67% of planned performance. I would use this as an internal measure of performance.


Have a look at Earned Value Management (EVM). There you compare your currently Earned Value (EV) (usually the cost that were planned to complete a specific task) with the actual cost (AC) for the task or the cost planned (PV for planned value) to be achieved to a specific point in time. Having those three numbers, you get performance indices:

Cost Performance Index CPI: EV / AC

Schedule Performance Index SPI: EV / PV

Your 15/10 formula provides the same message like the SPI. Due to the wide distribution of EVM, I recommend the 15/10 or SPI representation.

By the way, when you look for EVM, you'll discover that you get a lot more nice statistic values out of the 3 types of cost...

  • 1
    SPI becomes 1 at schedule completion. Earned Schedule would be a better set of metrics since the OP is asking about duration. In this case, the OP would look for ES, AT, SPIt, and iEACt values during the project. Commented May 9, 2015 at 11:38

Most answers here talk of "overruns". This presupposes that your estimate was somehow the "right" figure and that the failure occurred in not completing within this estimated time and thus the time taken was somehow "wrong". This is a destructive way of looking at things. What really went wrong was that the a poor estimate was made.

The correct estimate would have been 15 days. The actual estimate was 10 days, so the estimate was only 10/15 = 66.67% accurate.

  • 1
    An estimate is given as a range. A planning value is given as a single number. The OP did not provide the estimate so it is presumed 10 is within the estimate range. Also, one actual observation of 15 does not make 15 more accurate than 10. Ten could have been the most likely within the estimate and the actual result of 15 could have been an exceptional result. Commented May 11, 2015 at 9:58

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