Total float is what many of us are aware of, and is commonly referred to as a float.
Total float is the amount of time an activity can be delayed without delaying the project completion date. On a critical path, the total float is zero.
Total float is often known as the slack.
Total Float = Late Start date – Early Start date
or Total Float = Late Finish date – Early Finish date
Free float is the amount of time an activity can be delayed without delaying the Early Start of its successor activity.
You can calculate the free float by subtracting the Early Finish date of the activity from the Early Start date of next activity (ES of next Activity – EF of current Activity).
If two activities are converging to a single activity, only one of these two activities may have free float.