Slack Time Calculator — Total Float for Project Management
Enter Early Start, Early Finish and Late Finish for a project activity to calculate total slack (float) — how many time units the activity can be delayed without affecting the project deadline. Zero slack marks a critical-path activity.
The activity can slip this many time units without delaying the project
- 1
Activity duration
EF − ES = 5 − 0 = 5 - 2
Late Start (LS)
LF − duration = 10 − 5 = 5 - 3
Total slack
LS − ES = 5 − 0 = 5Zero slack means this activity is on the critical path.
How does this calculator work?
Total slack = LF − EF = LS − ES. Zero slack means the activity is on the critical path and any delay lengthens the project. Positive slack means scheduling flexibility. Given Early Start (ES), Early Finish (EF) and Late Finish (LF), compute: duration = EF − ES, Late Start LS = LF − duration, Slack = LS − ES.
Formula
How this is calculated
Slack time (also called total float) is a concept from the Critical Path Method (CPM). Each activity has four key times: Early Start (ES — the earliest it can begin given predecessors), Early Finish (EF = ES + Duration), Late Finish (LF — the latest it can end without delaying the project end), and Late Start (LS = LF − Duration). Total float is LS − ES (equivalently LF − EF): how much the activity can slip without pushing the project past its deadline.
Activities with zero float form the critical path — any delay on them directly delays project completion by the same amount. Project managers monitor critical-path activities most closely because there is no scheduling buffer. Activities with positive float are non-critical; they can start late or pause, but each day of delay consumes float and eventually turns the activity critical. Negative float means the plan is infeasible — the activity cannot finish within its LF given its duration.
This calculator derives the four CPM parameters for one activity in isolation. In a full network you would first run a forward pass (computing ES and EF from the project start for all activities) and a backward pass (computing LF and LS from the project end), then use those values here to check individual activities.
Frequently asked questions
Total float (computed here) is how much an activity can be delayed without pushing the project end date. Free float is the subset of that delay which does not push out the Early Start of any immediately following activity — it is always ≤ total float. An activity can have large total float but zero free float when a sensitive successor depends on its early finish.
Yes — negative slack means the current plan is infeasible: the activity cannot finish within its Latest Finish given its duration and start constraints. Negative slack is a trigger to crash activities (add resources to shorten duration), fast-track (overlap sequential activities), or renegotiate the deadline.
The filled portion of the bar represents the span from Early Start to Early Finish — the activity itself. The unfilled remainder from Early Finish to Late Finish is the slack: the buffer available before the activity becomes critical. A bar that fills the entire track means zero slack (critical path).
Also known as
TG we-Calculate Editorial Team. (2026). Slack Time Calculator — Total Float for Project Management [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/slack-time-calculator
TG we-Calculate Editorial Team. "Slack Time Calculator — Total Float for Project Management." TG we-Calculate. 2026. https://we-calculate.com/calculator/slack-time-calculator.
TG we-Calculate Editorial Team, "Slack Time Calculator — Total Float for Project Management," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/slack-time-calculator
@misc{wecalculate_slack_time_calculator, title = {Slack Time Calculator — Total Float for Project Management}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/slack-time-calculator}}, year = {2026}, note = {TG we-Calculate} }
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