# How to calculate the size of a project in the days-person unit of measurement?

Once in a while I have read here and there the size of a project expressed in a matter of `days-person` or `person-day`.

I may understand what this means, but I don't know on what do people base themselves to calculate it.

1. What are the variables considered into this calculation?
2. How these variables are used in the calculation formula?
3. Otherwise, how to estimate it grossly, when something is missing from the formula's variables?
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That's called guesstimation – user2567 Feb 18 '11 at 15:35
The only reliable way of doing this is to apply Hofstadter's law. – mouviciel Jan 30 '12 at 8:26
What do you mean by a person-day, specifically? The average amount of work one person can accomplish in a real workday, or the amount of work one person would accomplish in an undistracted workday? For anything other than short high-priority projects, there will be a difference. – David Thornley Jan 30 '12 at 17:56

Standard Agile way:

1. Divide project into user stories;
3. Have ppl who are actually going to work on the task estimate how much time they need;
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A Man-day Simply means 8 hours of work for 1 person.

1) Estimate how long it will take to perform each task

3) divide by 8

= Man-days

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Never ever divide by 8. You should allow only 6 hours of productive time perday in your estimate. You have to account for leave, unavoidable delay, administrative work, required meetings called by HR, etc. – HLGEM Feb 18 '11 at 18:18
No. 6 hours is not a Man-day. Calculating actual calendar time is diffrent. – Morons Feb 18 '11 at 18:20

Some variables to consider:

• admin work: How much administrative work do people on the project spend each day (or week)?
• meeting time: Do meetings count towards the same total or do you want it in a separate total?
• how much will the project change? Unless the initial requirements are perfect and carved in stone, expect there to be time spent changing things. Just how solid the requirements are at the begining can affect an estimate.
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