Primavera P6 Project Calculations: Resource Assignment – When Updating Actual Units or Cost

Before discussing progress updates and resource tracking in Primavera P6, one of the most overlooked critical steps is the Primavera P6 calculation options under the Projects window, such as the “Calculations” tab. These settings govern how the software behaves when actual data is entered, resources are assigned, or changes are made during execution. As shown in the screenshot, options such as “Add Actual to Remaining” or “Subtract Actual from At Completion” significantly impact how Primavera calculates remaining effort, at-completion values, and overall project forecasting.

Failing to configure these options correctly can lead to inaccurate earned value metrics or conflicts in reporting. This article will explore the “When updating Actual Units or Cost”, and what is the difference between “Add Actual to Remaining” and “Subtract Actual from At Completion“.

Primavera P6 Calculation Options

When updating project progress in Primavera P6, accurately reflecting the use of labor, cost, and materials is essential for reliable forecasting. The screenshot above illustrates two options available under Resource Assignments that directly affect how Primavera recalculates the At Completion values during updates:

1. Add Actual to Remaining

• This option tells Primavera to change  the “At Completion” value based on the actuals recorded.
• This is useful when actual productivity is lower than expected, and you want to capture the impact on future resource requirements.

Example

We have here “Activity A” with resources making the budgeted total cost 2,200 $, original duration 10 days, and the assigned Budgeted labor unit are 100.

If we updated the activity saying it started 7-May-2025 with 25% progress, or we can add progress to the Labour units manually, the “At Completion” will increase to 125 units. Meaning that Primavera P6 added the value of the update on the “At Completion” original value.

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If we changed the progress percentage to 75%, the “At Completion” value would increase to 175

In order to keep the “At Completion” fixed, you need to change the “Remaining” after updating, so in the above screenshot, we need to change the “Remaining” to 25 to have a fixed “At Completion” that equals 100.

2. Subtract Actual from At Completion

• This setting assumes that the “At Completion” value remains fixed unless manually changed.
• Primavera will automatically reduce the remaining units by the actuals entered.
• Suitable for projects where resource forecasts are more rigid or where re-forecasting is done manually by planners.

Example

Same activity, but now our calculation option in Resource Assignment is “Subtract Actual from At Completion”

If we updated the activity saying it started 7-May-2025 with 25% progress, the “At Completion” will remain 100 units. Meaning that Primavera P6 fixed the value of the “At Completion” and decreased the remaining based on the actual units.

If we changed the progress percentage to 84%, the “At Completion” value would still be fixed, and the remaining would be 16 units.

If you want to learn more about scheduling techniques, you can enroll in our Construction Project Planning Intermediate Level with P6 course.

Conclusion

Primavera P6 calculation options offer flexibility through its calculation settings, but that flexibility must be used wisely. It’s essential not to leave these settings at their defaults without review. Instead, you should align them with the specific requirements of your project and the expectations of your stakeholders. By doing so, you ensure that your baseline, progress updates, and monitoring efforts remain consistent, reliable, and truly reflective of your project’s real performance.

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