Project Management guide · Scheduling and time
Leads and Lags in Project Management: Examples and Formulas
A lag is a deliberate delay between two linked activities, such as waiting three days for paint to dry. A lead is the opposite: it lets a successor start before its predecessor finishes, overlapping the two. Both are applied to task dependencies.
Key takeaways
- A lag adds waiting time between linked activities; a lead overlaps them.
- Leads are written as negative lags and are a form of fast tracking.
- Do not use lags to hide work or contingency.
- Document why each lead or lag exists.
Lead vs lag
| Lag | Lead | |
|---|---|---|
| Effect | Adds waiting time between activities | Overlaps activities |
| Notation | FS+3 days | FS-3 days (a negative lag) |
| Typical reason | Curing, drying, notice periods, delivery time | Starting work early to save time |
| Schedule impact | Lengthens the path | Shortens the path |
| Risk | Low | Higher, because work starts on incomplete inputs |
Worked example: lag
A Pour concrete slab takes 2 days. B Build walls takes 5 days and cannot start until the concrete has cured for 3 days.
| Activity | Link | Start | Finish |
|---|---|---|---|
| A Pour slab | - | Day 0 | Day 2 |
| B Build walls | FS + 3 days | Day 5 | Day 10 |
The lag is not an activity: no one works during it and it uses no resources, but it still sits on the path, so the total is 2 + 3 + 5 = 10 days.
Worked example: lead
C Develop module takes 10 days. D Test module takes 6 days. Testers can start on finished features 2 days before development ends.
| Link | D starts | D finishes | Time saved |
|---|---|---|---|
| FS (no lead) | Day 10 | Day 16 | - |
| FS - 2 days (lead) | Day 8 | Day 14 | 2 days |
Using a lead is a form of fast tracking: it saves time but increases the risk of rework if late changes affect work already tested.
Leads and lags with other link types
- SS + 2 days: lay cable can start 2 days after trench digging starts.
- FF + 1 day: documentation finishes 1 day after the software finishes.
- SS + 50%: some tools let you express lag as a percentage of the predecessor's duration.
Common mistakes
- Using lags to represent real work, such as a supplier's manufacturing time. Model that as an activity so it can be tracked.
- Hiding contingency inside lags, which makes the schedule hard to understand.
- Large negative lags that make the logic confusing; consider splitting the predecessor instead.
- Forgetting whether a lag is in working days or calendar days. Concrete cures on weekends; most people do not work them.
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Frequently asked questions
What is a lag in project management?
A specified delay between a predecessor and its successor, such as FS+3 days.
What is a lead in project management?
An amount of time a successor can start before its predecessor finishes, often written as a negative lag such as FS-2 days.
Is a lead the same as a negative lag?
Yes. Most scheduling tools record a lead as a negative lag.
Do lags use resources?
No. A lag is waiting time, not work. If resources are needed, create an activity instead.
Are leads and lags on the critical path?
They can be. A lag on a critical link adds directly to the project duration.
When should I use a lead?
When the successor can safely start on partial output from the predecessor, usually to shorten the schedule.
What is an example of lag time?
Waiting 7 days after sending a consultation letter before the response period closes.
Is lag the same as float?
No. Lag is a planned delay built into a link; float is flexibility calculated from the schedule.
Can lags be in calendar days?
Yes. Many tools let you specify elapsed (calendar) days for lags such as drying or curing time.
Do leads increase risk?
Usually yes, because the successor starts with incomplete information and may need rework.