If you break a project down into tasks, estimate those tasks, and then add up all the estimates to figure out how long the project is going to take it will be wrong. Usually the estimates will be off, but also you’re unlikely to have thought of everything when you first break down the work.

If you don’t add some slack into the project to account for this then you’re very likely to miss the original estimate. The slack will cover things like team members sick leave, triaging and fixing critical bugs, as well as discovering you’ll need to do more work than you thought at the start.

With enough slack you can handle all this and still deliver when you said you would, or earlier. Without it every time something happens the project gets delayed a little bit and then nobody trusts the estimates any more.

What have you estimated recently and how much slack did you build into the estimate?