Difference between Calendar Days and Work Days

By Mainak Biswas September 08, 2006 - 1,030 views

Both seasoned IT entrepreneurs as well as developers make this common mistake. They don’t seem to quite understand the difference between calendar days and work days. So let me ask you this question – “What should be the delivery date of a project starting on October 1st and having an estimated effort of 160 hours?”

A typical calculation goes like this:

160 hrs / 8 hrs per day = 20 days. So, if the project is starting on October 1st, it should all be done on 21st October. Right? Wrong!

Here is what is involved, in understanding the delivery date based on start date and total number of hours that is required to accomplish the task:

Number of Days per Week
This is the number of days for which the office of the developer’s organization is going to be open in a week. It can be any number from 5 – 6. Like some companies are open for half time on Saturday. So, this figure is 5.5 for such an organization.

Number of Hours per Day
This signifies, the number of normal work hours that developer will be working in particular day. Do not take overtime in account because then you will not have any buffer to use should the project slip.

So, based on the above two numbers, if the Number of Days per week is 5 and the number of hours per day is 8 then the developer is going to work 8 hours per day then you will 8 X 5 = 40 hours of work done per week.

This means it will take 4 weeks to get 160 hours of work done. This is an equivalent of 28 calendar days which means that given a project started on 1st October it cannot be completed before 28th October.

Granted, that the above calculation is very simple, but you get the message! In reality, things are more complex because the utilization of hours depends on other factors like Number of resources on the project, Number of parallel tasks, Holidays etc.

Why knowing this is important?

Knowing this is very important for two very important reasons:

1) It forces you to plan in advance. As you try to think about delivery dates, you automatically begin thinking about the things that affect it.

2) It prevents unnecessary dissatisfaction. Had you not known this, the delivery on October 28th would have been perceived as a 25% slippage in project deadline whereas the developer has been able to meet the deadline.

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