Calculator Inputs
This tool supports finish-to-start, start-to-start, finish-to-finish, and start-to-finish relationships. It also compares actual lag with a target lag and tolerance.
Example Data Table
These sample project links show how lag behaves under different dependency relationships.
| Predecessor Task | Successor Task | Dependency | Anchor Pair | Actual Lag | Interpretation |
|---|---|---|---|---|---|
| Excavation | Concrete Pour | FS | Finish → Start | 1.50 days | Normal lag for inspection and cleanup. |
| Design Draft | Peer Review | SS | Start → Start | 0.50 days | Review begins after initial drafting starts. |
| Hardware Setup | Commissioning | FF | Finish → Finish | 2.00 days | Commissioning completes later than setup. |
| Data Migration | User Training | SF | Start → Finish | -0.75 days | Lead used to compress final handoff. |
Formula Used
Lag time depends on the chosen dependency type. Negative values represent lead. Positive values represent lag.
- FS: Lag = Successor Start − Predecessor Finish
- SS: Lag = Successor Start − Predecessor Start
- FF: Lag = Successor Finish − Predecessor Finish
- SF: Lag = Successor Finish − Predecessor Start
- Variance: Actual Lag − Target Lag
- Working-Day Conversion: Working Days = Working Hours ÷ Workday Hours
When working-day mode is selected, the calculator counts only the chosen number of working days per week and converts overlap into working hours.
How to Use This Calculator
- Enter predecessor and successor task names.
- Select the dependency type that matches the relationship.
- Fill in the start and finish times for both tasks.
- Choose working-day or calendar-day display mode.
- Set workday hours, weekly workdays, target lag, and tolerance.
- Press Calculate Lag Time to show the result above the form.
- Review the graph, lag classification, and status note.
- Use the CSV or PDF buttons to export the result.
Frequently Asked Questions
1) What is lag time in project management?
Lag time is the intentional gap between linked tasks. It measures the time separating the predecessor milestone and the successor milestone defined by the chosen dependency.
2) What is the difference between lag and lead?
Lag is a positive delay between related tasks. Lead is a negative gap that overlaps work and compresses the schedule, which can increase coordination risk.
3) Why do dependency types matter?
Each dependency uses a different anchor pair. FS compares finish to start, SS compares start to start, FF compares finish to finish, and SF compares start to finish.
4) When should I use working days instead of calendar days?
Use working days when weekends or nonworking days should not influence the displayed lag. Use calendar days when the full elapsed time matters for reporting.
5) Can this calculator handle negative lag?
Yes. A negative result means lead. That indicates the successor milestone occurs before the predecessor anchor, creating overlap between connected activities.
6) What does variance mean here?
Variance compares actual lag against your target lag. A positive variance means the gap is wider than planned. A negative variance means it is tighter than planned.
7) Why add tolerance days?
Tolerance creates an acceptable range around the target lag. It helps planners separate normal variation from situations that need schedule review or corrective action.
8) What does the graph show?
The graph compares actual lag, target lag, variance, and tolerance in one view. It makes over-buffering, compression, and review thresholds easier to spot quickly.