CPM scheduling and float: total vs free float

The four dates and two floats

For each activity you work out four dates:

• Early start (ES): the largest early finish of the activities before it. The first activity starts at 0.
• Early finish (EF): ES + duration.
• Late finish (LF): the smallest late start of the activities after it. The last activity's LF is the project finish.
• Late start (LS): LF - duration.

Then:

• Total float = LS - ES (or LF - EF). How long the activity can slip without delaying the project finish.
• Free float = the earliest ES of the next activities - this activity's EF. How long it can slip without delaying any following activity.

Activities with zero total float are critical. Free float can never be more than total float.

A worked schedule with invented durations

Forward pass:

• A: 0 to 3
• B: 3 to 8
• C: 3 to 5
• D: 8 to 12
• E: 5 to 11
• F: starts at the later of D (12) and E (11), so 12 to 15

The project takes 15 days.

Backward pass from day 15:

• F: LF 15, LS 12
• D: LF 12, LS 8
• E: LF 12, LS 6
• B: LF 8, LS 3
• C: LF is E's late start, 6, so LS 4
• A: LF is the smaller of B's LS (3) and C's LS (4), so 3, and LS 0

Float:

The critical path is A, B, D, F.

Check by paths: A-B-D-F is 3 + 5 + 4 + 3 = 15 days. A-C-E-F is 3 + 2 + 6 + 3 = 14 days. The difference, 15 - 14 = 1 day, is the total float on C and E, which matches the table.

Float belongs to the path

C and E share the same single day. If C slips 1 day, E's early start moves to 6 and E has no float left. That is why C's free float is 0 even though its total float is 1: any slip in C pushes E. Many contracts treat float as shared by the project; read the contract before relying on it in a delay claim.

Other terms you will meet

• Negative float: the schedule shows the work finishing after a required date. The difference is negative total float, and something must be crashed or resequenced.
• Crashing: adding resources to shorten critical activities. Shortening a noncritical activity does not shorten the job.
• Lag: a required wait between activities, such as cure time before stripping forms.

Common mix-ups

• Using the smallest early finish on the forward pass. At a merge, take the largest.
• Using the largest late start on the backward pass. At a burst, take the smallest.
• Treating each activity's float as separate. Activities on the same path share it.

On the NASCLA exam

Scheduling is part of General Requirements, 25 of the 115 scored questions. Construction Project Management is a listed reference that fits CPM, so tab its scheduling topic. Draw the network on scratch paper and check the critical path by adding each path, as above.

Practice it

Try the float question in the NASCLA practice test. Schedule submittal lead times into the network with submittals and RFIs, and see how time is added in change orders. Check your own network with the free CPM float calculator. Contract time usually runs from the notice to proceed, and a late finish can cost liquidated damages.

Take the free 10-question check

Start the free 10-question check. It is weighted to the twelve NASCLA subject areas, needs no signup and explains every answer, so you see which area to study first.

The NASCLA Commercial course has a lesson for each of the 12 subject areas that shows which listed reference to open and how to apply it, original practice questions with every answer worked, and four timed practice exams of 115 questions on a 330-minute clock. Your scores come back by subject area, so you know where the next study hour goes. See the course.

Not affiliated with or endorsed by NASCLA or PSI. NASCLA and PSI are named only to identify the exam and its test vendor.

Sources

Show the list

• PSI Candidate Information Bulletin 1458, content outline and reference list, updated 7 Jul 2026. Retrieved 5 Nov 2026.
• NASCLA Commercial Exam, NASCLA. Retrieved 5 Nov 2026.