Negative float, also known as negative slack, is the amount of time beyond a project’s scheduled completion that a task within the project requires.
Total float is the amount of time a task within a project can be delayed without endangering the project deadline. Float is often represented as a positive or negative number representing the number of days of delay.
Essentially, negative float is the amount of time that must be saved to bring the project to completion on time. Negative slack can also indicate a scheduling problem when, for example, a task’s start date is set earlier than the end date for a preceding task in the critical path. Because each task in the critical path must be completed before the next can be started, if there is negative float, it may be necessary to find unused float elsewhere in the project plan.
See a video demonstration of calculating float: