One of the things that McConnell's "Software Estimation" repeats a lot is that estimates are _intervals_. I'm a bit surprised not many people here have mentione

One of the things that McConnell's "Software Estimation" repeats a lot is that estimates are _intervals_. I'm a bit surprised not many people here have mentioned that. If you give a single number, there's a 100% chance that it's wrong -- you're never going to hit it exactly. Instead, what you should be aiming for is to give an interval that is correct, say, 90% of the time (so still wrong 1 out of 10 times). If you do it honestly, the intervals have to be much wider you than you are used to -- the "two weeks" you would usually say is realistically more between 8 working days and six weeks, with 90% confidence. A major benefit of giving intervals to people instead of fixed numbers is that it makes it really clear that it's not a deadline and can't be treated as one. If they tell you this is unacceptable, then only give the high number, and McConnell's book.