Slack, more formally called float, is the amount of time a scheduled task can be delayed before something else breaks. There are two answers to that question and they are almost always different numbers. Free float is how long the task can slip before its immediate successor has to move. Total float is how long it can slip before the project finish date has to move. Confusing the two is one of the most reliable ways to lose a week without noticing.
This slack time calculator from Arb Digital takes the four figures a critical path network already produces — earliest start, duration, latest finish, and the earliest start of the successor — and returns both floats, the interfering float between them, and a plain-language verdict on whether the task sits on the critical path. It also tests a specific slip you are thinking about, so the answer is not abstract.
What This Slack Time Calculator Does
Enter the task's earliest start and duration and the tool computes its earliest finish. Enter the latest finish from your backward pass and it computes the latest start. From those it derives total float as latest start minus earliest start, which is identical to latest finish minus earliest finish. Free float comes from the successor: the earliest start of the earliest successor, minus any lag on the link, minus this task's earliest finish.
Interfering float is the difference between the two. It is the part of a task's slack that belongs to the path rather than to the task — spend it and the successor moves even though the project end date does not. Naming it separately matters, because that is exactly the float that gets consumed silently and then blamed on the next task in line.
How to Use It
- Work in day numbers, not dates. Day 0 is the project start. Converting to calendar dates afterwards avoids every weekend and holiday problem inside the arithmetic itself.
- Take earliest start from the forward pass. It is the latest of the earliest finishes of all predecessors, plus any lag.
- Take latest finish from the backward pass. It is the earliest of the latest starts of all successors, minus any lag. If you have not run a backward pass, you do not yet have a float figure of any kind.
- Use the earliest successor for free float. If several tasks depend on this one, the constraint is whichever starts soonest.
- Test the slip you actually face. Enter it in the last field and read the verdict rather than eyeballing the floats.
How Float Is Calculated
The four relations are short. Earliest finish = earliest start + duration. Latest start = latest finish − duration. Total float = latest start − earliest start = latest finish − earliest finish. Free float = successor earliest start − lag − earliest finish, floored at zero.
With the default figures: earliest start 10, duration 6, so earliest finish is 16. Latest finish 20, so latest start is 14. Total float is 14 − 10 = 4 days, and the cross-check gives the same answer, 20 − 16 = 4. The earliest successor starts on day 18 with no lag, so free float is 18 − 16 = 2 days. Interfering float is 4 − 2 = 2 days. A three-day slip therefore stays inside the project's total float but overruns the free float by one day, which means the successor has to move even though the project end date does not — and that is precisely the case a task owner will report as "we absorbed it".
Why Total Float Is Not the Task Owner's to Spend
Total float belongs to a path, not to a task. If four tasks sit in series on a path with four days of total float between them, that is four days in total for all four of them, not four days each. The first task to slip consumes it and every downstream task on that path is left with none. The float has not vanished; it has been transferred.
This is why experienced schedulers treat float as a shared, depletable project resource and require a decision before it is used, rather than letting each owner discover independently that they had "some slack". The moment a path's total float reaches zero, that path becomes critical, and a task nobody was watching starts driving the finish date. Recomputing float after every progress update, not just at the start, is what catches that handover.
The consequence for reporting is direct. A status report that says a task is "three days late but has float" is not enough information. The questions are which float, how much of the path's float remains after the slip, and whether any other task on that path was relying on it.
Negative Float and What It Actually Means
If latest finish is earlier than earliest finish, total float is negative. That is not an error in the arithmetic — it is the schedule telling you the network cannot meet an imposed constraint. Negative float almost always appears when a deadline or a milestone date has been forced into the plan without shortening the work that feeds it.
The number is useful because it quantifies the gap. Four days of negative float means four days of duration must be removed from that path, or four days of relief must be negotiated on the constraint. It is not fixed by re-baselining the dates and it is not fixed by asking for effort; it is fixed by changing durations, changing logic, or changing the date. A plan carrying negative float on any path is, formally, a plan that already misses its date.
Float, Buffers and the Difference Between Them
Float is calculated. A buffer is inserted. They behave very differently under pressure. Float falls out of the network logic and moves whenever durations or dependencies change, so it is an honest, live reading of how much room a path has. A buffer is a deliberate block of time added by a planner, and it survives until someone decides to spend it.
Problems start when buffers are hidden inside task durations rather than shown as their own line. Padding every estimate by 20% produces a schedule with plenty of apparent float and no visibility into where the protection actually sits, and the padding tends to be consumed automatically because work expands to fill the estimate. Keeping estimates honest and putting protection in named buffers gives the same total duration with far better information. The PERT calculator is the usual way to produce those honest three-point durations before any of this float arithmetic begins.
The Boundary With Lead Time, PERT and Earned Value
Several tools on this site work on adjacent parts of the same problem, and it is worth being precise. This page answers one question: how much can this task slip. The PERT calculator answers a different one, how long the task is likely to take, using optimistic, most likely and pessimistic estimates. The earned value calculator answers a third, how far ahead or behind the whole project is against its plan, in cost and schedule terms — the discipline behind which is set out at length in the GAO Cost Estimating and Assessment Guide.
The lead time calculator deals with elapsed time in a supply chain rather than float in a project network — a procurement lead time is often an input to a task duration, but it has no float of its own until it sits inside a network. For the labour side, the project profitability calculator and the billable hours calculator handle the money that a schedule slip eventually turns into.
Calendars, Constraints and the Things That Break Float
Float arithmetic assumes the network is the only thing controlling the dates. Two common practices quietly break that assumption. The first is hard date constraints — "must start on", "must finish on" — which override the logic and can create float figures that look reasonable while describing a schedule the network no longer drives. The second is resource levelling, which moves tasks to fit people and equipment. After levelling, a task with apparent total float may in fact be unable to move at all, because the only person who can do it is booked elsewhere.
Different calendars on linked tasks are a third source of confusion: a task on a five-day calendar feeding one on a seven-day calendar produces float measured in different kinds of day at each end. None of these make float useless, but they do mean a float figure should be read alongside the constraints and the resource plan, not on its own. Guidance on documenting exactly this is set out in the GAO Schedule Assessment Guide, which treats a properly sequenced network with justified durations as a precondition for any credible schedule analysis. For converting day numbers back into working dates, our business days calculator is the simpler route.
Arb Digital builds and maintains a large library of free, no-signup business and technical calculators. Everything is browsable in one place, and we are easy to reach if you have a question.
Browse All Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Treating total float as belonging to one task — it is shared along the whole path, and the first task to slip spends it for everyone.
- Reporting a slip as absorbed because total float covered it — if free float was exceeded, the successor still moved.
- Calculating float once and never again — float changes with every progress update, and yesterday's non-critical path can be today's critical one.
- Hiding buffers inside task durations, which creates float that looks real but is actually padding nobody can see or manage.
- Ignoring negative float — it is a measurement of how far the plan already misses its date, not a rounding artefact.
Related Free Tools From Arb Digital
Estimate durations first with the PERT calculator, track progress against the plan with the earned value calculator, and cost the outcome with the project profitability calculator or the billable hours calculator. On the supply side, the lead time calculator handles elapsed procurement time and the business days calculator converts day numbers into working dates. The full free online tools hub lists everything else.
Frequently Asked Questions
Slack, or float, is the amount of time a task can be delayed before something else has to move. Free float measures the delay before the successor moves; total float measures the delay before the project finish date moves.
Total float is latest start minus earliest start, which is always equal to latest finish minus earliest finish. With an earliest start of day 10 and a latest start of day 14, total float is four days.
Free float is the delay a task can absorb without moving its successor. Total float is the delay it can absorb without moving the project end date. Free float is never larger than total float, and the gap between them is interfering float.
In a network with no imposed date constraints, tasks with zero total float form the critical path. Where deadline constraints exist, the critical path can carry negative float instead, so the rule is the lowest float value in the network rather than zero specifically.
It means the network cannot meet a date that has been imposed on it. Four days of negative float means four days must be removed from that path's durations or logic, or four days of relief negotiated on the constraint.
No. Float is calculated from the network logic and changes whenever durations or dependencies change. A buffer is time a planner deliberately inserts and it stays until someone decides to spend it.
Yes. Resource levelling and hard date constraints both override network logic, so a task can show total float while the person or equipment needed for it is unavailable at any later point.
This page performs standard critical path arithmetic on the values you enter and holds no schedule of its own. Float figures are only as good as the network logic, durations and constraints behind them, and any real schedule decision should be made against your own maintained project schedule.
