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Total float and Gantt charts

Float tells you how much an activity can be delayed without delaying the whole project. A Gantt (cascade) chart shows this at a glance.

Total float

Total float = latest finish time - duration - earliest start time. Earliest start = early time of the start event; latest finish = late time of the end event. Critical activities have zero float. Show all three numbers in your working.

Gantt (cascade) chart

Draw a time axis. Put critical activities in a row along the top. Below, draw each non-critical activity as a bar starting at its earliest start time, with its float shown as a dotted or shaded extension. The bar can slide right within the float.

Reading the chart

At a given time, an activity must be happening if it cannot avoid that time: latest start ≤ t and earliest finish ≥ t + 1 for the interval from t to t + 1. It may be happening if it could be scheduled across that time. Read the scale carefully: activities run from the start of one time unit to the start of another.

Limits

A Gantt chart shows timing and flexibility but not dependence: delaying one activity within its float can reduce the float of activities after it.

Worked example

Find the total float of activity D in project J.

  1. Earliest start of D = 4
  2. Latest finish of D = 14
  3. Float = 14 - 5 - 4 = 5

Answer: Total float of D = 5 days

Key idea: Float = latest finish - duration - earliest start. Critical activities go along the top of a Gantt chart; bars slide within their float.

Check yourself

  1. Find the total float of activity E in project J.
    Show answer

    3 days. 14 - 4 - 7 = 3.

  2. Find the total float of activity B in project J.
    Show answer

    1 days. 7 - 6 - 0 = 1.

  3. Find the total float of activity I in project J.
    Show answer

    3 days. 19 - 3 - 13 = 3.

  4. Which activities of project J must be happening during the interval from time 7 to time 8?
    • F, D, E
    • E
    • B, F
    • F
    Show answer

    F. Must happen: latest start ≤ 7 and earliest finish ≥ 8. Only F qualifies; D and E could also be scheduled then, but have enough float to avoid it.

  5. On a cascade chart, where are the critical activities drawn?
    • At the bottom with their floats
    • In a single row along the top
    • In a column on the left
    • Anywhere, with no float shown
    Show answer

    In a single row along the top. Critical activities fill the whole project time end to end, so they form one row along the top.

  6. Activities D and G in project J are in sequence and each shows float on the Gantt chart. If D is delayed by 3 days, what happens to G?
    • Nothing happens to G
    • The project is delayed
    • G becomes a dummy
    • G's available float is reduced
    Show answer

    G's available float is reduced. D has float 5 and G has float 3; some of D's float is shared with G, so delaying D uses up float that G could have used. The project is not delayed while the delay stays within D's float.

  7. What does a total float of zero tell you?
    • The activity can start at any time
    • The activity is critical: any delay delays the project
    • The activity takes no time
    • The activity is a dummy
    Show answer

    The activity is critical: any delay delays the project. Zero float means no slack at all.

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Last updated 7 November 2026. Spotted a mistake? Tell us.