Example Data Table
| Activity |
Duration |
Predecessors |
Meaning |
| A |
4 |
- |
Starts the project network. |
| B |
6 |
A |
Starts after activity A finishes. |
| C |
5 |
A |
Runs after A and may run beside B. |
| D |
3 |
B|C |
Starts after B and C are both complete. |
| E |
4 |
C |
Depends only on C. |
| F |
2 |
D|E |
Finishes the sample network. |
Formula Used
Early Start: ES = maximum EF of all predecessor activities.
Early Finish: EF = ES + activity duration.
Late Finish: LF = minimum LS of all successor activities.
Late Start: LS = LF - activity duration.
Total Float: TF = LS - ES, or LF - EF.
Free Float: FF = minimum successor ES - current EF.
Independent Float: IF = minimum successor ES - maximum predecessor EF - duration.
Critical Activity: An activity is critical when total float equals zero.
How to Use This Calculator
Enter one activity per line. Add the activity name first. Add the duration second. Add predecessors third. Leave the predecessor field blank for starting activities.
Use the pipe symbol for multiple predecessors. For example, D,3,B|C means activity D takes 3 units and starts after B and C finish.
Choose the delimiter that matches your pasted data. Set the unit name, decimal places, and project start offset. Press Calculate to show the full forward and backward pass table.
Understanding Early and Late Activity Times
Purpose of the Network
Project networks show how work flows from one activity to another.
Each task has a duration and a dependency link.
The calculator reads those links and builds a valid sequence.
It then performs a forward pass and a backward pass.
These passes help planners see the first and last acceptable time for each task.
This makes the schedule easier to control.
Forward Pass
The forward pass starts with activities that have no predecessors.
Their early start equals the project start offset.
Their early finish equals early start plus duration.
For later tasks, the early start is the largest early finish among predecessors.
This rule protects logic.
A task cannot start until all required earlier work is complete.
The largest early finish at the end becomes the project finish time.
Backward Pass
The backward pass works from the project finish back to the first task.
Final activities receive the project finish as their late finish.
For earlier tasks, late finish is the smallest late start among successors.
Late start equals late finish minus duration.
This shows how late each activity may begin without delaying the whole project.
It also exposes weak points in the schedule.
Float and Critical Work
Float measures scheduling freedom.
Total float shows the delay allowed before the project end moves.
Free float shows the delay allowed before the next activity moves.
Independent float is stricter.
It checks freedom after considering both predecessors and successors.
Interfering float shows float that may affect later activities.
An activity with zero total float is critical.
Critical activities form the controlling route through the network.
Delaying any critical activity usually delays the project.
Noncritical activities still matter.
Their float can disappear when estimates change.
Review the table after every schedule update.
Use the exported files to share results with teams.
Keep activity names short and unique.
Check missing predecessors before trusting the final plan.
FAQs
What are early activity times?
Early activity times are the earliest possible start and finish times. They are found with a forward pass through the project network.
What are late activity times?
Late activity times are the latest start and finish times allowed without delaying the project finish. They are found with a backward pass.
What does total float mean?
Total float is the amount of time an activity can slip without delaying the whole project. Zero total float marks critical work.
What does free float mean?
Free float is the time an activity can slip without delaying the early start of its immediate successor activity.
How do I enter multiple predecessors?
Use the pipe symbol between predecessor names. For example, F,2,D|E means F depends on both D and E.
Can the calculator handle multiple start activities?
Yes. Any activity with no predecessor is treated as a start activity. Each receives the chosen project start offset.
Can the calculator detect invalid networks?
Yes. It checks missing predecessors, duplicate activities, invalid durations, self-dependencies, and circular dependency logic.
What does a critical path candidate mean?
It is a connected chain of critical activities. Large networks may have more than one controlling route with zero total float.