Your question is an NP-Complete problem:) There are many algorithms used to come up with an estimate but they are always just guesses, are never accurate, and many take a long time to execute. Forget hour estimates, use scrum or some other agile framework. Making estimates for a project in hours at its start is simply lying to people.
Don't make hour-based estimates until right before you build the feature and update those estimates continually as you progress on the feature.
Don't forget to include time for testing in your estimates.
