Dear PM Advisor. February 25, 2013

Dear PM Advisor,

What's the difference between Free Float and Total Float? 

Floater in New York, NY

Dear Floater,

I assume you know what float is. This is also known as slack, the amount of time an activity can be delayed without delaying the overall end-date of the project. Free Float is similar in that it is the amount of time an activity can be delayed without delaying the next activity.

American Eagle Group describes the difference quite well in this post. Look at the picture they use:

Notice that free float can only occur at the end of a series of non-critical path activities.

For those who love math, here are the formulae for Total Float and Free Float:

Total Float = LS - ES  or  LF - EF
Free Float = ESs - EFp - lag  (Where ESs means Early Start of the successor activity. EFp means Early Finish of the Predecessor activity)

(By the way, this last formula only works if you use the zero day method for calculating ES and EF. Otherwise you get the number 1 for Free Float on every activity.)

Looking at the network diagram above, you can see that total float is 0 for the critical path and 11 for activities C, D and E. Any time you delay one of these tasks, you eat up some of that Total Float.

Since you can start D right after C, even if C is delayed, there is no Free Float on C. Same reason there is no Free Float on D. But since you have to wait for Activity G to complete before you can start Activity H there is float on Activity E unless C, D and E delay more than 11 days. So the Free Float only appears on the last activity of this chain.

Good luck,

PM Advisor

Send your questions to bfieggen@gmail.com