Learn · Scheduling 101
Float: how much room a piece of work has, and what it is measured against
After this subject you can read a float number and say what it is measured against, tell the two kinds apart, and know why one of them never goes below zero here.
How much can this one slip?
Every piece of work in a schedule carries two answers from the last lesson: the soonest it can go, and the latest it may run before something gives. The gap between those two is room. Knowing how much room a piece has is the difference between a worry and a decision, and it is the whole of this lesson.
Room measured against the project finish
Total float — working days this activity can slip before the project finish slips. Those are Brikt's own words for it, on the column header, and it is the number most people mean when they use the word at all. Slack is the other word for float; schedulers use both and mean the same room.
Why it matters — it is counted in working days, so ten days of room is ten days the project calendar counts as working, not ten days on the wall.
Room measured against the next piece of work
Free float — working days this activity can slip before the next dependent activity slips. Same unit, different question. The first asks what the project can stand; the second asks what the work immediately after can stand. A piece can have three weeks of the first and none at all of the second.
When the number goes below zero
Set a ceiling the work in front of it cannot reach — a finish-no-later-than the logic cannot meet — and the two do not agree, which is where the last lesson left off. Brikt does not move the date to make it fit: it works the schedule out anyway, and the total float on that run of work comes out below zero.
Where the textbooks and this product differ
In scheduling generally, free float can go below zero: a bound on the work that follows, or work done out of sequence, can put its earliest date ahead of where this piece can finish. In Brikt it cannot. The Free Float column has a floor at zero, so it never shows a number below it.
And the rule that usually holds between the two
Free float is never larger than total float — as long as the backward pass is seeded at or after the finish the forward pass worked out. Brikt's floor breaks even that: free float never goes below zero, so on a row whose total float IS below zero, free float is above it. Read the two as two questions, not a ranking.
Why it matters — the two columns disagreeing that way is not a defect. It is the floor doing what it was built to do.
What this lesson does not settle
Work with no room left gets talked about as if it answered a larger question about the job. Not everyone agrees: whether having no room makes a piece of work the thing deciding the finish is answered in more than one way, and this page does not settle it. The lesson after this one is where that argument lives.
Reading the number out loud
Ask
How many days of room does this one have, and against what?
Instead of
That one has plenty of time.
Ask
Is that room this piece's alone, or shared with everything behind it?
Instead of
We have two weeks on that.
Ask
Why is this number below zero?
Instead of
It has a minus sign — skip it.
What to remember
- Total float is measured against the project finish; free float against the next piece of work.
- Slack is the same idea under the other name, and schedulers use both.
- A TOTAL float below zero means a ceiling and the work in front of it do not both hold.
- Free float can go below zero in scheduling generally, and never does in Brikt's own column.
- It is never larger than total float when the backward pass is seeded at or after the forward pass's finish — except in Brikt, where the zero floor puts it above on any row whose total float is below zero.
Read the room on a real schedule
Open a project, show the Free Float column beside the Total Float one, and read a few rows across. Pick one with plenty of room and one with none, and say out loud what each of the two numbers is measured against. If you would rather do this on your own job, bring a schedule to a demo and we will read it together.