Gantt Chart vs Critical Path: What Each One Actually Tells You

These two are constantly treated as rivals, and they are not. A Gantt chart is a way of drawing a schedule. The critical path is a calculation performed on that schedule. You can have a beautiful Gantt chart with no idea what your critical path is — and that is the single most common reason a project slips without anyone noticing until it is too late.

The one-line difference

A worked example: a 9-task website migration

Nine activities, durations in working days, with the predecessors each one depends on. This is the raw table you would type into any scheduling tool before it draws a single bar.

Step 1 — forward pass (early dates)

Walk forward through the network. Early Start of a task is the latest Early Finish among its predecessors; Early Finish = Early Start + duration. Day 0 is the morning of day one.

The project takes 33 working days. That number is the only thing a Gantt chart gives you for free — and it tells you nothing about which tasks produced it.

Step 2 — backward pass and float

Now walk backwards from day 33. Late Finish of a task is the earliest Late Start among its successors; Late Start = Late Finish − duration. Total float = Late Start − Early Start. Zero float means the task is critical.

The critical path is A → B → C → E → H → I, 33 days long. Content rewrite (D) can slip three days for free. CMS setup (F) can slip seven. Visual design (C) cannot slip by a single day without moving the go-live date.

Step 3 — what each view now tells the room

• The Gantt chart tells the sponsor: design finishes mid-month, content load in week five, cutover on day 33. It is the picture that gets signed off.
• The critical path tells you: freeze the design review calendar, because C is critical and a two-day approval delay is a two-day launch delay.
• Float tells you where the slack lives: the content writer being pulled onto another project for three days is survivable; the front-end developer being pulled for one day is not.
• Near-critical warning: D → G carries only 3 days of float. One bad week and it becomes the critical path. Monitor anything within 10% of the critical duration.

Compressing the schedule

Once you know the critical path, compression becomes arithmetic rather than argument. Crashing means adding resources to critical tasks only — money spent on D or F buys you nothing at all. Fast tracking means overlapping critical tasks, for example starting the front-end build (E) on the approved header and navigation designs before all of C is complete, accepting rework risk in exchange for days.

That third row is the whole point of this article. Without the float calculation, adding a writer looks like progress. With it, you can see it moves nothing.

Which should you build first?

1. 1List activities and durations — no dates yet.
2. 2Record real predecessors, and be honest about which are genuine constraints.
3. 3Run the forward and backward passes to get the critical path and float.
4. 4Only then draw the Gantt chart, and mark the critical path in a distinct colour.
5. 5Re-run the calculation every time an actual finish date differs from the plan — the critical path moves during delivery.