Estimates are guesses about how much work something will be. Humans are optimistic in nature, so tend to underestimate how much work is required.

I tend to estimate in man days, but this is always full time man days that does not equate to elapsed time. Elasped time must take into account holidays, sickness, meetings, interuptions. At most, allow to 80% of effort a week, i.e. 4 working days out of a 5 day week.

Software does not scale like making a factory, you cannot ask ten developers to do ten days effort in one day.

Don't forget to include:

• unit tests
• integration tests
• migration
• builds
• deployment
• hardware requirements/lead times
• DR

How much contingency should you allow? Research suggests software projects require around 160% to allow for 80% certainty of completion on time. Will that amount of contingency be acceptable to your clients? Of course not.

If asked "why is the project late?" always respond with "I'm sorry, we underestimated, we did not allow enough contingency". The project is not late, the estimate was wrong.