# How to Use Leads and Lags in Project Schedule?

## Leads and Lags – Definitions, Explanations & Examples

Leads and Lags are essential for making a good project schedule. They are the basic building blocks of scheduling. Many scheduling experts don’t use them. Instead, they often apply (incorrect) alternatives. In my opinion, Leads and Lags are easy to use and implement. They should always be used whenever they are required.

I am writing a two-part article to explain the concept of Leads and Lags. In this part, I will describe the basics of the concept with the help of diagrams & examples. In the next part, I will remove confusions related to the concept by answering a few Frequently Asked Questions.

I think, you would be already familiar with Precedence Diagramming Method (PDM), Project Network Diagrams and Project Dependencies. It is important to understand these concepts before reading further. You should read my articles on Precedence Diagramming Method and Project Dependencies before venturing ahead.

There are 4 Logical Relationships in PDM – Finish to Start, Start to Start, Finish to Finish and Start to Finish. Leads and Lags are modifiers of these 4 logical Relationships. Let us understand the concept with the help of diagrams & examples.

### What is Lead?

#### Definition

The amount of time whereby a successor activity can be advanced with respect to a predecessor activity.

PMBOK Guide 5th Edition

A Lead provides acceleration to the Successor Activity. You can also look at Max Wideman’s Glossary for some other definitions of Lead.

#### Example

Let us consider two activities A and B.

• Duration of A – 3 days
• Duration of B – 2 days
• B has a Finish to Start Relationship with A with a Lead of 1 day.
• The scheduled Start of B is 1 day before the scheduled Finish of A.

#### Project Network Diagram Representation of Lead

In our example, the Project Team would need 4 days to complete these activities. If the said FS relationship was without any Lead, the Project Team would have needed 5 days.

#### Mathematical Representation of Lead

The scheduled Start(S) of B is 1 day before (-1d) the scheduled Finish(F) of A.

B(S) = A(F) – 1d

#### How to Use and Apply Leads?

• A – Collect Requirements, B – Create Design.
• A – Interior work of a Building, B – Exterior landscaping of the Building.

### What is Lag?

#### Definition

The amount of time whereby a successor activity is required to be delayed with respect to a predecessor activity.

PMBOK Guide 5th Edition

A Lag provides mandatory delay to the Successor Activity. You can also look at Max Wideman’s Glossary for some other definitions of Lag.

#### Example

Let us consider two activities A and B.

• Duration of A – 3 days
• Duration of B – 2 days
• B has a Finish to Start Relationship with A with a Lag of 1 day.
• The scheduled Start of B is 1 day after the scheduled Finish of A.

#### Project Network Diagram Representation of Lag

In our example, the Project Team would need 6 days to complete these activities. If the said FS relationship was without any Lag, the Project Team would have needed 5 days.

#### Mathematical Representation of Lag

The scheduled Start(S) of B is 1 day after (+1d) the scheduled Finish(F) of A.

B(S) = A(F) + 1d

#### How to Use and Apply Lags?

• A – Lay ceiling for Floor I, B – Construct Columns for Floor II.
• A – Ship goods to a customer, B – Check if shipment has been delivered.

The concept of Leads and Lags has many practical applications in Project Scheduling. Sometimes they are mandatory because of the Project Constraints or environmental reasons. At other times they are introduced to lower the Project Risk or because of stakeholders’ demands.

You should also read second part of this article that answers 9 Frequently Asked Questions on Lead and Lag. I hope, after reading these two posts, you would be able to apply Leads and Lags in your project schedule. Please leave a comment if you have question.

#### Related Articles

9 Frequently Asked Questions on Lead and Lag

Project Network Diagram

Project Constraints

Project Risk

What are Project Dependencies?

4 Types of Project Dependencies

### 2 thoughts on “How to Use Leads and Lags in Project Schedule?”

1. Edison M Braun says:

What do you think about the effects of leads and lags on a quantitative risk analysis?How can we associate uncertainties with them, as they may exist..
We are having lots of discussions about it right now in our company.

• Praveen Malik says:

Hi Edison – You can use Monte Carlo analysis.