How to Calculate Float in Project Management
A no-nonsense guide to float, free float, and the critical path method, with a free calculator built right into this page.
Ever built out a project schedule and run into the word float? Some people call it slack instead. It sounds like jargon, but it’s actually pretty simple: float is just how much a task can slip before it starts messing with your whole timeline.
This guide walks through what float really means and how you’d work it out by hand. There’s a free tool further down the page too, so you don’t have to run the numbers yourself unless you want to.
What is float in project management?
Float is the amount of time a task can be delayed without affecting something else, either the next task or the final project deadline. Understanding float helps you know which tasks you can be flexible with, and which ones you cannot afford to touch.
There are two types of float you need to know:
Total float
Total float is the maximum amount of time a task can slip without delaying the overall project end date. Tasks with zero total float are on the critical path. Any delay to them delays the whole project.
Free float
Free float is the maximum amount of time a task can slip without delaying the very next task in the schedule. It’s a smaller, more localized cushion than total float.
The critical path method (CPM)
Float comes out of the Critical Path Method, a scheduling technique where you map every task, how long it takes, and what it depends on. CPM works the numbers two ways:
- Forward pass: works out the earliest a task could start (ES) and finish (EF), going from day one of the project to the end.
- Backward pass: works out the latest a task can start (LS) and finish (LF) without pushing the deadline, going from the end back to the start.
Once you’ve got those four numbers, working out float is just subtraction.
Float formulas
| Metric | Formula |
|---|---|
| Total float | Late Start (LS) − Early Start (ES) |
| Total float (alt.) | Late Finish (LF) − Early Finish (EF) |
| Free float | Early Start of next task − Early Finish of current task |
A simple worked example
Say your project has three tasks. Task A takes 3 days and starts the project. Task B (4 days) and Task C (2 days) can only start once A is finished.
- Task A: ES 0, EF 3, LS 0, LF 3 → total float = 0 (critical)
- Task B: ES 3, EF 7, LS 3, LF 7 → total float = 0 (critical)
- Task C: ES 3, EF 5, LS 5, LF 7 → total float = 2 days
Task C can be delayed by up to 2 days without affecting the project’s finish date. Tasks A and B cannot slip at all. They sit on the critical path.
Free float & critical path calculator
Plug in your own tasks, durations, and dependencies below, and it’ll work out total float, free float, and the critical path for you.
Frequently asked questions
What is the difference between float and slack?
They mean the same thing. “Float” is more common in construction and engineering projects, while “slack” is the term used in Microsoft Project and some software tools.
Can float be negative?
Yes, it can. If float goes negative, the project’s already running late, either the deadline needs to move, or something in the schedule needs to be compressed.
Why does the critical path matter?
It’s the longest chain of dependent tasks in the whole project. Slip anything on it, and the entire project slips too, so it’s usually the first place to look when you’re trying to save time.
Do I need special software to calculate float?
Not really. Small projects are easy enough to work out by hand, or with the calculator above. Once things get bigger, most people switch to something like Microsoft Project or Primavera P6.
For larger, enterprise-level projects, many organizations in the UK bring in specialist consultancies. Turner & Townsend, for example, offers dedicated project management services for complex builds and infrastructure work.