Critical Path Calculator
A free critical path method (CPM) calculator that runs entirely in your browser. Enter activities, link them with FS/SS/FF/SF relationships and lags, and get the forward and backward pass, total float, the critical path and a Gantt chart — recomputed as you type, on a real working calendar. No sign-up, no activity limit, nothing uploaded.
House extension
CPM schedule — start 2026-07-20 — sitraka.lu/checklists/scheduler
| ID | Activity | Type | Dur (d) | Predecessors | Early start | Early finish | TF (d) | |
|---|---|---|---|---|---|---|---|---|
| Mon 20 Jul 08:00 | Mon 20 Jul 08:00 | 0.0 ● | ||||||
| Mon 20 Jul 08:00 | Tue 21 Jul 17:00 | 0.0 ● | ||||||
| Wed 22 Jul 08:00 | Fri 24 Jul 17:00 | 0.0 ● | ||||||
| Mon 27 Jul 08:00 | Fri 31 Jul 17:00 | 0.0 ● | ||||||
| Thu 23 Jul 08:00 | Mon 27 Jul 17:00 | 4.0 | ||||||
| Mon 03 Aug 08:00 | Mon 03 Aug 17:00 | 0.0 ● | ||||||
| Tue 04 Aug 08:00 | Mon 17 Aug 17:00 | 0.0 ● | ||||||
| Tue 18 Aug 08:00 | Fri 21 Aug 17:00 | 0.0 ● | ||||||
| Mon 10 Aug 08:00 | Mon 17 Aug 17:00 | 4.0 | ||||||
| Mon 24 Aug 08:00 | Fri 28 Aug 17:00 | 3.0 | ||||||
| Mon 24 Aug 08:00 | Wed 02 Sept 17:00 | 0.0 ● | ||||||
| Wed 02 Sept 17:00 | Wed 02 Sept 17:00 | 0.0 ● | ||||||
Predecessors: B (finish-to-start), B:SS+2 (start-to-start, 2-day lag), B:FF-1, B:SF+3. Lags are in working days on the successor's calendar. Rows highlighted in red are on the longest path. Press Enter in the last row to add an activity.
Project finish
Wed 02 Sept 17:00
Duration
33 working days
Longest path
9 / 12 critical
Critical path
A → B → C → D → F → G → H → K → L
12 activities scheduled in 4 ms, entirely in your browser. Your data is saved locally and never uploaded: the “Copy link” button encodes the whole project into the URL itself.
Worked examples, step by step
Three networks solved in full — forward pass, backward pass, float and the critical path — each one preloaded in the calculator so you can change a duration and watch the answer move.
Forward and backward pass: a worked example
Six activities, finish-to-start links only, on a Monday-to-Friday 8-hour calendar starting Monday 7 September …
Start-to-start lags and finish-to-finish links: a worked example
The same six activities you would find in a foundation pour, but now overlapped. Formwork starts two days afte…
Working days vs calendar days: the same schedule, two answers
Four sequential activities, eight days of work in total, starting Thursday 10 September 2026. Nothing about th…
What people use it for
Construction look-aheads
Sketch a two-week look-ahead or sequence a small works package with real FS/SS/FF logic before it goes into the master schedule.
Maintenance shutdowns
Turnarounds run around the clock — switch to the 24/7 calendar and check the critical chain of an isolation-inspect-repair-reinstate sequence.
IT cutovers & migrations
UAT, defect fixing overlapping with SS lags, training in parallel, a go/no-go milestone: see which stream actually drives the go-live date.
Learning CPM without a licence
Asked to learn scheduling but no seat available? Practice link types, lags, floats and longest-path reasoning here — the conventions match what assessors expect.
Events & deadlines
Work backwards from a conference date or an exam: finish milestones, FF-linked marketing, and float tell you what can slip and what cannot.
Teaching & interviews
Build a small network live, break it (cycle detection shows the loop), move a lag and watch float and criticality react — instant feedback beats slides.
Matching examples are one click away in the “Load an example…” menu above — construction, a 24/7 shutdown, an IT cutover, a conference and an exam plan.
The conventions, done properly
Toy CPM demos count durations in naive calendar days. This one is built on a minute-accurate scheduling engine that follows the conventions planners know from professional tools such as Oracle Primavera P6®:
- durations are worked minutes on the activity's calendar — weekends never count;
- a start lands on the next calendar opening, a finish on the previous closing;
- FF and SF links pull the successor's finish, and its start is derived backwards;
- total float is measured in working time, per activity calendar;
- the critical path is traced through driving links (longest path), not inferred from float alone;
- circular logic is detected and reported with the exact loop.
The engine behind this page is covered by 60+ unit and property-based tests and schedules 10,000 activities in under 50 ms.
FAQ
- Why does my first activity start Monday at 08:00?
- Durations are counted in working time on the activity calendar. A start is projected to the next calendar opening and a finish to the previous closing — the same edge conventions professional scheduling tools use. On the default 5×8 calendar the first opening after a weekend is Monday 08:00.
- How is total float computed?
- A forward pass computes early dates, a backward pass computes late dates, and total float is the working time between them (LS − ES on the activity's own calendar). Critical activities are traced structurally along the longest path through driving links — not just flagged by float ≤ 0.
- Is my schedule uploaded anywhere?
- No. The scheduling engine is compiled into this page and runs in your browser. Your activities are saved to your browser's local storage only — nothing leaves your machine.
- Can I share a schedule with someone?
- Yes — the Copy link button encodes the whole project into the URL fragment. Whoever opens the link sees your schedule, yet nothing is stored on any server: the data travels inside the link itself.
- Can I use this to practice for a P6 assessment or interview?
- That is one of its main uses. The engine follows the same scheduling conventions (working calendars, four link types with lags, total float, longest path), so the reasoning you practice here transfers. For the software's own menus and layouts you will still need the real thing.
- What about resource loading, baselines and XER files?
- This free page is deliberately small: activities, four link types with lags, two calendars, floats, a Gantt, CSV export and share links. A fuller planning app — XER import, editable Gantt, baselines, multi-calendar projects — is in the works on the same engine.
Want more than a page?
XER import, an editable Gantt, baselines and data-date progress are being built on this same engine. If you'd like early access — or want to tell me what a web scheduler must get right — write to hello@sitraka.lu.
Oracle and Primavera are registered trademarks of Oracle and/or its affiliates. This free tool is an independent project and is not affiliated with, sponsored by or endorsed by Oracle. Results should be verified before contractual use.