At times, there may be two project activities that have the potential to be carried out simultaneously, but the second activity can only begin after a certain delay following the initiation of the first activity. The minimum required delay is usually defined by the technological process.

Traditionally, such delays are represented in a schedule using Start-to-Start plus Duration Lag (SS + Lag) logic.

However, this approach has a significant planning issue. If activity ‘A’ has started but is not progressing as expected, the schedule incorrectly indicates that activity ‘B’ can start as planned, when in reality it cannot. 

The delay could be due to external factors or lower than expected progress.

Activity ‘B’ only can start after the planned volume of work is achieved, which is in 3 working days.

An alternative approach is to use VOLUME as the primary measure of activity and calculate the duration based on the productivity of assigned resources. This method offers several advantages, including the accurate simulation of activity lag duration.

Task A:

  • Volume: 100 meters
  • Resource productivity: 20 meters per day
  • Duration: 5 days

Task B:

  • Volume: 100 meters
  • Resource productivity: 20 meters per day
  • Duration: 5 days

In this case, lag is defined as the minimum required volume between activities based on the planned or actual progress of the predecessor activity.


  • Volume: 40 meters
  • Duration: 2 days

If the first activity has started but has not reached the planned volume, the start of the second activity will be automatically rescheduled.

Furthermore, if a resource with a different productivity rate is assigned to the work, the scheduling system should recognise that both the activity duration and lag duration differ from the original plan.

Task A
• Volume = 100 meters
• Resource Productivity = 40 meters per day
• Duration = 2.5 days
• Lag = 40m
• Lag Duration = 1 days

When using Duration Lags in scheduling, there is a significant risk that the overall duration will not be accurately calculated. This is because the planner must remember that the lag duration depends on the assigned resource and manually make adjustments every time there is a change in the assignment.

Volume lag can be beneficial for various scenarios, not just for (SS + lag) dependencies. For example, it can also be used for (FF + lag) dependencies.


Also, Volume Lag is applicable to Negative Lags (Leads).

Volume lag could be measured in volume units or as % of overall volume.

Not all activity lags can be measured in terms of volume. There are situations where lags are directly defined by duration. However, when a technological process specifies a lag based on volume, but the schedule represents the lag using duration as the primary measure, it becomes difficult to simulate the schedule scenario accurately.

By incorporating volume lag, it becomes possible to calculate the duration of the lag correctly, and it can also be dynamically adjusted based on resource assignment and activity progress.

Alex Lyaschenko

PMO | Portfolio Planning & Delivery | PMP | P3O Practitioner | AgilePM Practitioner | Six Sigma