Type in your activities, durations and predecessors. The forward pass, backward pass, early and late dates, total float, free float and the critical path itself update as you go — with a chart showing where the slack actually sits.
| Activity ID | Description | Duration | Predecessors |
|---|
Red bars are on the critical path. The thin grey line after a bar is that activity's total float — how far it can slide right before the finish date moves. Durations are working days; this calculator uses finish-to-start logic with no lags, no calendars and no resource levelling, which is the model behind every worked CPM example.
The critical path is the longest chain of dependent activities through a project — and therefore the shortest possible time the project can take. Every activity on it has zero float: delay any one of them by a day and the finish date moves by a day. Everything off the critical path has slack, and delaying it costs you nothing until that slack runs out.
That is why the critical path is the only sensible place to spend management attention. Crashing an activity with 20 days of float buys you nothing. Recovering a day on the critical path buys you a day.
Walk the network from start to finish. An activity's early start (ES) is the latest early finish of all its predecessors; its early finish (EF) is ES plus its duration.
The largest EF in the whole network is the project duration.
Now walk back from the finish. An activity's late finish (LF) is the earliest late start of its successors — or the project duration if it has none. Its late start (LS) is LF minus duration.
Every activity with zero total float is on the critical path. Chain them together and you have it.
| Measure | Answers | Whose problem it is |
|---|---|---|
| Total float | How long can this slip before the project finishes late? | The client's date |
| Free float | How long can this slip before the next activity is pushed? | The next subcontractor |
Free float is never larger than total float. An activity with 15 days of total float and zero free float looks safe on a report and is not — the moment it slips, someone downstream is standing around, even though the completion date holds for a while longer.
Float is shared, not owned. Two activities in sequence with 10 days of total float between them do not have 10 days each — they have 10 days between them. The first one to use it takes it from the second. This is the single most common misreading of a float column in a status report.
The example loaded above is a small plant installation. Two chains leave the design activity: council approval → earthworks → foundations, and a 20-day procurement of long-lead plant. Both converge on the installation. Which one is critical depends entirely on the durations — change the procurement from 20 days to 45 and watch the critical path jump to the other branch, and the earthworks chain suddenly acquire float. That switch is the reason a critical path has to be recalculated every period rather than agreed once.
An activity with 2 days of float is not "safe" — it is one bad week from being critical. In practice, treat anything under about 10 days of float on a construction programme as a second critical path and manage it accordingly. A schedule where the second-longest path is a day shorter than the longest one has, for practical purposes, two critical paths.
Forward pass for early dates, backward pass for late dates, then total float = LS − ES. Every activity with zero total float is on the critical path.
Total float is slip before the project finishes late. Free float is slip before the next activity is pushed. Free float is never greater than total float.
Yes — any two chains of identical length are both critical. It makes the schedule more fragile, because there are more places a delay can hurt.
An imposed date sits earlier than the network can deliver. It measures how late you already are before anything else goes wrong. Pure CPM with no imposed dates cannot produce it.
Both are used and float is identical either way. Day 0 is continuous time, which is what scheduling software uses; day 1 is whole-day numbering, which most exam questions use. The toggle above switches between them.
No — it is deliberately the textbook finish-to-start model, so its numbers match the worked examples people check against. For a real schedule with lags, calendars and constraints, run the DCMA 14-point assessment on your P6 or MS Project export instead.
Aegis reads your P6 or MS Project schedule each period and tracks what the critical path did — which activities drove the slip, where float was consumed, and what it means for the completion date — then writes it up for the client.
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