Is this the same as critical-path slack?

The concept is related, but this tool compares two totals entered directly, not the phase-by-phase slack a dependency graph produces. See the Phase Dependency Planner for that instead.

What does negative slack actually mean in practice?

It means the work genuinely requires more time than is available, a real shortfall that needs to be closed through more time, more capacity, or reduced scope, rather than an error in the numbers.

Schedule Slack Calculator

Enter how much time is available and how much time the work requires. The difference is your slack, positive for a buffer, negative for a shortfall.

Measuring hours of capacity against demand rather than days of schedule? See Capacity Gap Calculator.

Want to learn more?

Critical Path Basics: Why Some Delays Slip the Deadline

Worked example

24 days of required work against 20 available days leaves negative 4 days of slack, a real 4-day shortfall that has to be closed some other way, whether that means more time, more capacity, or reduced scope. Flip those same numbers, 20 days of required work against 24 available days instead, and the result comes back as positive 4 days of slack, a genuine buffer rather than a shortfall.

Common mistakes

Reading zero slack as automatically safe. Zero slack means the schedule is exactly full, with no room left for any delay at all.
Treating negative slack as a calculation error rather than a real signal. It means the numbers entered genuinely do not leave enough time as they stand.

Limitations

This takes both numbers as given directly. It does not calculate available or required time from an actual calendar, so pairing it with a business-day or deadline calculator covers that separate step.

Frequently asked questions

Is this the same as critical-path slack?: The concept is related, but this tool compares two totals entered directly, not the phase-by-phase slack a dependency graph produces. See the Phase Dependency Planner for that instead.
What does negative slack actually mean in practice?: It means the work genuinely requires more time than is available, a real shortfall that needs to be closed through more time, more capacity, or reduced scope, rather than an error in the numbers.