P6 Schedule Calculator

Primavera P6 schedules dates from activity durations, relationships, lags and working calendars. Checking those dates by hand helps you understand why an activity starts when it does, validate a contractor's programme or plan quick what-if scenarios. This calculator computes start and finish dates using working days and relationship logic.

How to use it

1. Enter the start date and the working calendar, for example 5 or 6 working days per week.
2. Enter the activity duration in working days.
3. Add relationships with predecessors and any lag.
4. Read the calculated early start and early finish.

Relationship types

• Finish to Start (FS): the successor starts after the predecessor finishes. The default and most common.
• Start to Start (SS): the successor starts after the predecessor starts, plus lag.
• Finish to Finish (FF): the successor finishes after the predecessor finishes.
• Start to Finish (SF): rare, used for just-in-time handovers.

Worked example

Formwork starts Monday 2 November 2026 with a 5-day duration on a Monday to Friday calendar, so it finishes on Friday 6 November. Concrete pour follows with FS and a lag of 0: it starts Monday 9 November. Curing then must last 7 calendar days before stripping: if modelled as an FS lag of 7 on a 7-day calendar, stripping can start Monday 16 November.

Things that often surprise people

• Lag calendars: P6 can measure lag on the predecessor's, successor's, 24-hour or project calendar. Curing is usually a calendar-day lag.
• Holidays in the calendar push dates out.
• Constraints like Start On or After can override logic and create negative float.
• Long lags hide work; consider modelling waiting time as a separate activity.