Estimate the size of the product (number of lines of code or function points). Some projects jump right into estimating the schedule itself, but effective estimation requires estimating the size of the software to be built first. This step is by far the most difficult intellectually, which might be part of the reason that people so often skip it.

Estimate the effort (man-months). If you have an accurate size estimate and historical data on your organization's performance on similar projects, computing the effort estimate is easy.

Estimate the schedule (calendar months). Once you've estimated the size and effort, for reasons explained later in the chapter, estimating the schedule turns out to be nearly trivial. But getting a realistic schedule estimate accepted can be the most difficult part of the project. I devote the entire next chapter to that topic.

Wrapped around these three steps is a more general step, a meta-step:

Provide estimates in ranges and periodically refine the ranges to provide increasing precision as the project progresses.