Lags and leads

A dependency says which task waits for which. A lag says how long it waits, and a lead says how far it may jump ahead. In Microsoft Project both live in the same small Lag box, and both are easy to type and hard to see afterwards. That is why owners should care about them: a seven-day lag between a slab pour and the next trade can be sound engineering or a place where a contractor has parked float. This lesson shows how to enter lags and leads, how working and elapsed lags differ, and how to keep them honest in an owner's master schedule.

Learning outcomes

• Explain what lag and lead do to a dependency, and enter both in Microsoft Project desktop.
• Choose between working-day lag, elapsed lag and percentage lag, and predict the dates each gives.
• Read lag codes such as FS+7ed and SS+50% in the Predecessors column.
• Set owner rules for when a lag is allowed and how it must be documented.

The lesson

Lag and lead are properties of the link

In Lesson 7 you linked tasks with finish-to-start (FS), start-to-start (SS), finish-to-finish (FF) and start-to-finish (SF) dependencies. A lag or lead is not a task and has no row of its own. It is a number stored on one dependency. Microsoft Support describes lead time as an overlap between two tasks and lag time as a delay between them, and its first instruction is that the dependency must already exist [1].

Figure 1 shows the three cases on one FS link. With no lag, the successor starts as soon as the predecessor finishes. A positive lag pushes the successor out. A negative lag, which Project calls lead, pulls the successor back so the two tasks overlap.

Where to type it

There are three places, and they all write to the same field:

• Task Information. Double-click the successor's name to open Task Information, select the Predecessors tab and type the value in the Lag column [1]. This is the method Microsoft documents and the one to teach new schedulers.
• The link line. In the Gantt Chart, double-click the arrow between two bars to open the Task Dependency box, which has the link Type and a Lag box. It is quick, but on a dense chart you can easily pick the wrong arrow.
• The Predecessors column. Type the code straight into the cell. The format is the predecessor's ID, the dependency type, then a signed duration: Microsoft's own example is 14FS+3d, task 14 finish-to-start with a three-day lag, and it says to type a negative number for lead [2]. Several predecessors are separated by the list separator, normally a comma [2].

The Predecessors column is also the best review tool. Insert it next to Duration in the Entry table and every lag in the file is visible in one scan.

Three kinds of lag

Working-time lag is the default. Type 3d and Project counts three working days on the schedule's calendars, so weekends and the Ontario statutory holidays you built in Lesson 4 do not count. Use it when the waiting period is really a working period, such as an inspection booked three business days after a request.

Elapsed lag runs around the clock and ignores the calendar. You add an "e" to the unit, exactly as for the elapsed durations covered in Lesson 6: 7ed is seven elapsed days, or 168 hours. Elapsed time counts every hour of every day [4]. Use it for physical processes that keep going on weekends: concrete curing, sealant cure, a mandatory notice period counted in calendar days.

Percentage lag is a share of the predecessor's duration. Microsoft's example is -50%, which lets the successor start when the predecessor is about half done [1], and a typed code looks like 2FS-50% [3]. The lag is recalculated when the predecessor's duration changes. That is useful when the overlap really is proportional, and confusing when nobody remembers the lag is a percentage.

Lead is negative lag, and it is a forecast

A lead lets work overlap, and construction overlaps constantly: roofing starts on one bay before the last wall panels are up. The problem is that FS-10d is a claim about the future. It says the successor can start ten days before the predecessor finishes, whatever happens to the predecessor. If the predecessor slips and is half done instead of nearly done, the lead still fires ten days before its forecast finish. An SS link with a lag, which ties the successor to the predecessor's start, often describes the same overlap more honestly. Better still, split the predecessor into two tasks at the point where the successor can really begin, and link FS from the first part.

Common schedule-quality checks, such as the DCMA 14-point assessment used in North American project controls, flag every lead and limit how many links may carry lags. You do not have to adopt those thresholds, but the reasoning holds: a lead is hard to verify, and a large lag hides work that should be a task.

Northgate worked example

Northgate Distribution Centre (fictional) is a 12,000 m² tilt-up warehouse in Vaughan, Ontario, for Northgate Logistics Inc. (fictional), built by Halvorsen Builders Ltd. (fictional) under CCDC 2-2020. The owner keeps its master schedule in Microsoft Project desktop as NDC_OwnerMaster_2027-01-11.mpp, on the Ontario calendar from Lesson 4, with the summary WBS from Lesson 5. All facts come from the Northgate fact sheet; durations and lag values below are illustrative.

Curing before panel casting

Tilt-up panels are cast on the building slab, so in WBS 4 the first panel-casting task cannot start until the slab-on-grade in WBS 3 has cured. Milestone M4, foundations and slab complete, is Fri May 14, 2027. Suppose the structural engineer's requirement is seven days of curing (an illustrative value: the real figure comes from the specification). The scheduler links Slab-on-grade to Tilt-up panel casting FS and must choose 7d or 7ed.

Figure 2 shows why the choice matters. With 7d, Project counts seven working days: May 17 to 21, skips the weekend and Victoria Day on Mon May 24, then counts May 25 and 26. Casting starts Thu May 27. With 7ed, the lag runs from Friday at 5:00 p.m. for 168 hours to Fri May 21 at 5:00 p.m. The next working time is after the weekend and the holiday, so casting starts Tue May 25. Curing does not stop for Victoria Day, so 7ed is the right model, and it is two working days earlier.

View the data

Link	Lag counts	Lag ends	Panel casting starts
Slab-on-grade to panel casting, FS+7d	May 17–21, 25, 26 (working days)	Wed May 26, 2027, 5:00 p.m.	Thu May 27, 2027
Slab-on-grade to panel casting, FS+7ed	Fri May 14, 5:00 p.m. + 168 hours	Fri May 21, 2027, 5:00 p.m.	Tue May 25, 2027 (after weekend and Victoria Day)

Screenshot to come

A percentage lag that moves

Halvorsen's draft logic links Tilt-up panel erection to Roof joists and deck SS+50%: the roof crew starts when half the erection time has passed. The planned erection is 20 working days, so the lag is 10 days. Figure 3 shows what happens if erection takes 30 days instead: the percentage lag grows to 15 days and the roof start moves out with it, while a fixed SS+10d would start the roof on day 10 regardless. Which one is right depends on the erection sequence. If the roof crew needs a set number of bays standing, the percentage lag tracks reality better. If the crane plan fixes the roof start, the fixed lag does. The owner's job is to make the contractor say which, in the narrative.

View the data

Erection duration	Link	Lag in days	Roof starts on
20 days	SS+50%	10	Day 10
20 days	SS+10d	10	Day 10
30 days	SS+50%	15	Day 15
30 days	SS+10d	10	Day 10

The lag register

The owner's scheduler inserts the Predecessors column, filters for tasks whose Predecessors text contains "+" or "-", and copies the result into a short lag register: link, value, unit type and the reason. Each entry needs a source: a specification section, a supplier's lead time or the contractor's method statement. Anything over two weeks gets a second look, because a long lag is usually a task (approval, fabrication, delivery) that should be on the schedule with its own owner and duration.

Screenshot to come

Owner's move

• Write the lag rules into the schedule specification. No leads in the owner's master. Lags allowed only with a stated reason, recorded in the narrative or a lag register.
• Specify the unit. Curing and notice periods in elapsed days; work-based waits in working days. Ask which is which before the baseline is accepted.
• Turn long lags into tasks. "Steel shop drawings: Ridgeline review" is a 10-day task with an owner; a 10-day lag is invisible and cannot be statused.
• Review percentage lags every update. They move silently when durations change. Ask for them to be flagged in the update narrative.

Common mistakes

1. Using lag to hold a date. A lag that exists to land a task on a target date is a disguised constraint. When upstream work moves, the lag keeps the gap and the target date moves anyway.
2. Working-day lag for curing. It adds weekends and holidays to the wait and makes the schedule longer than the physics requires.
3. Leads on FS links that should be SS. FS-10d fires ten days before the predecessor's forecast finish, however far behind it is. SS with lag, or a split task, is clearer.
4. Forgetting the sign. Typing 5d when you meant -5d delays a task by ten days relative to what you intended, and the Gantt bar looks plausible either way.

Quick quiz

1. Where does Microsoft Support say to type a lag or lead in Project desktop?

   1. Task Information, General tab, Duration box
   2. Task Information, Predecessors tab, Lag column
   3. Project Information, Lag box
   4. Change Working Time, Exceptions tab

2. The code 14FS+3d in the Predecessors column means:

   1. Task 14 is a successor with 3 days of lead
   2. Finish-to-start from task 14 with a 3-day lag
   3. Start-to-start from task 14 with a 3-day lag
   4. The task lasts 14 days plus 3 days

3. A slab finishes Fri May 14, 2027 at 5:00 p.m. With FS+7ed on the Northgate calendar, the successor starts:

   1. Fri May 21
   2. Mon May 24
   3. Tue May 25
   4. Thu May 27

4. A link is SS+50% from a 20-day task. The predecessor's duration changes to 30 days. The lag becomes:

   1. 10 days
   2. 15 days
   3. 20 days
   4. Unchanged; percentage lag is fixed when entered

5. Which is the owner's best response to a 15-day lag labelled "approval period"?

   1. Accept it; lags are the contractor's business
   2. Replace it with a lead
   3. Ask for an approval task with a named reviewer and duration
   4. Change it to elapsed days

Show answers and explanations

1. B. Open Task Information, select the Predecessors tab and type the value in the Lag column [1].
2. B. The format is predecessor ID, dependency type, then signed lag; 14FS+3d is Microsoft's example [2].
3. C. 168 hours ends Fri May 21 at 5:00 p.m.; the weekend and Victoria Day are non-working, so the next working time is Tue May 25.
4. B. Percentage lag is a share of the predecessor's duration, so 50% of 30 days is 15 days [1][3].
5. C. A long lag usually hides real work. An approval task can be assigned, statused and argued about; a lag cannot.

Key terms

Lag

A positive delay on a dependency between the predecessor's driving date and the successor's.

Lead

A negative lag that lets the successor overlap the predecessor.

Elapsed lag

Lag in continuous time (for example 7ed), counted 24 hours a day and ignoring working calendars.

Percentage lag

Lag expressed as a share of the predecessor's duration, recalculated when that duration changes.

Predecessors column

The task field that lists each predecessor's ID, dependency type and lag as a code such as 14FS+3d.

Lag register

An owner's list of every lag in a schedule with its value, unit and justification.

References

1. Microsoft. Add lead or lag time to a task. Microsoft Support (Project desktop).
2. Microsoft. Predecessors (task field). Microsoft Support.
3. Teachucomp. Set Task Lead and Lag Time in Project: Instructions.
4. Dale Howard. Thinking About Elapsed Durations. MPUG.

Verified on November 6, 2026.

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