What does this change actually cost?

Edit the scenario, never the baseline. The finding is the knock-on: the activities that moved because of a change nobody made to them.

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What this change actually costs

Edit the scenario, never the baseline. The output is what moved — including the activities you did not touch.

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A planning aid, not a commitment. Everything here is calculated from figures you typed. A schedule is only as good as its estimates and its dependencies, and neither of those is something arithmetic can check for you. Treat the output as a way to test a plan, not as a promise to anyone about a date.

Baseline — the agreed plan

This is never modified by the analysis.

Scenario — the plan with the change in it

Side by side

What moved that you did not edit

Every difference

See the baseline as a Gantt →
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How the comparison works

Both plans get a full CPM pass. The baseline result is retained and never overwritten, and the report is the difference between the two.

Knock-on effects are separated from direct edits. An activity whose duration or dependencies you changed is a direct edit. An activity that moved without being touched is a knock-on, and that list is the reason to run this rather than reason about the change in your head.

Total float is compared as well as the finish date. A change that holds the date by consuming every day of slack has not been absorbed — it has been deferred, and the next surprise lands directly on the end date.

Never let the scenario become the baseline

The most common failure in change control is not analytical, it is procedural: the proposed plan gets saved over the agreed one, and a fortnight later nobody can answer "compared to what?"

That question is the only one anyone asks about a change after the fact. Keeping the baseline immutable is what makes the answer available, which is why nothing here writes back to it.

The knock-on is the point

Anyone can see that adding three weeks of work adds three weeks somewhere. What nobody can do reliably in their head is trace which other activities move, which lose their slack, and whether a different chain becomes critical.

A change that looks small because it touches one activity can cost a month because that activity was feeding two others through a chain nobody was thinking about. The list of things that moved without being edited is the finding.

Float is the shock absorber

A change that leaves the finish date untouched can still make the project substantially riskier by consuming the slack that was protecting it. The date looks fine and every remaining activity now sits on the critical path.

Reporting only the date movement lets that pass unnoticed, which is how a project absorbs four "no impact" changes and then slips on the fifth.

This costs schedule, not money

The comparison covers duration, dependencies, float and the critical path. It says nothing about cost, resourcing, quality or whether the change is a good idea — those need judgement and different data, and a tool that scored them would be inventing numbers.