# Critical Path Forward Pass Calculation - Start at Day Zero or One?

Critical Path calculation can start at Day Zero (0) or Day One (1). One of the most frequently asked and hotly debated topics about Critical Path calculation by PMP aspirants is whether to start the forward pass calculation at Day 0 or 1. This has been a topic of debate on numerous forums and blogs, including this blog. But I’ve not seen a conclusive answer anywhere. The PMP exam prep guides and courses add to the confusion by following different approaches. Some authors prefer to start at zero, some start at one. Each approach yields different results.

While practically speaking, it may be a matter of choice, but from the exam point of view, there’s only one correct approach. So, what is that correct approach? Should we start the forward pass calculation at Day zero or Day one? Today I’m going to answer that question for you, support it with evidence and put an end to this debate. So, are we ready to dive in?

## Illustration of the Problem

First, let’s consider a simple example and see how the results differ when we follow one approach over the other. Refer to figure 1 and 2.

### Figure 1 (Approach 1 - start at zero)

### Figure 2 (Approach 2 - start at One)

In figure 1, we started with zero and performed the forward and backward pass calculations. In figure 2, we started with one. Refer to the post Total Float vs Free Float for float formulas and calculations shown in the above examples.

### Key observations on the two approaches

- In both the cases, the Critical Path duration and float calculations yield the same result.
- The formulas to calculate the ES, EF, LS, LF, and FF are different.
- For calculating the duration of critical path and float, it doesn’t matter which approach you take as long as you use the right formula.
- The values of ES, EF, LS and LF are different in the two approaches.
- The formulas and calculations are relatively simpler in approach 1 (but your preference may be different).

Let’s look at the approach that some of the popular study guides and . Rita Mulcahy’s PMP Exam Prep uses approach 1, whereas Head First PMP and PM Prepcast use approach 2. There are some books that completely avoid both approaches and use a calendar date as the start date.

## What is the Correct Approach for the PMP Exam?

So, the (million-dollar) question is, which approach is right from the **PMP exam** point of view? Which approach should I use in the PMP exam?

**Update 04 March 2017**: This article was first posted about 5 years ago (Mar 2012). Based on the feedback in the comments and other changes that have happened since then, I’m updating this post. There’s no official word from PMI as to what the correct approach is. If I have to pick one of the two approaches at this point, I would go with Approach 2 as that’s what is used in the PMBOK^{®} Guide, 5th Edition.

For the PMP exam, Approach 2 - Start at Day 1 is the correct approach.

If you have questions or comments, please feel free to post them below. I’m always eager to read your comments. While we are here, I also want to invite you to join my new forum PM Hangout. All I’ll say is if you love this blog, you’ll love the forum too. So, what are you waiting for?

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