*This excerpt from the Kindle pocket guide, Practical Estimation, discusses the relative merits of the most common units used in relative estimation*.

# Abstract Estimation Units

When estimating the traditional way, a discussion of estimation units doesn’t happen very often. If someone asks you how long it will take to complete a task, they’re thinking in terms of calendar time. The implication is that they expect you to answer in kind.

But it doesn’t mean that you have to *estimate* in calendar time. Relative estimation necessarily means that you’ll be working in abstract units. Some examples of this are:

**Ideal Time**- An ideal day is a work day free from interruptions and other overhead.**T-Shirt Sizes**- Some teams use**S**mall,**M**edium,**L**arge and e**X**tra**L**arge to indicate the time required to complete projects.**Points**- Abstract units representing relative time to complete units of work.

When I first started practicing relative estimation, I was taught to think in terms of *ideal time*. If, for example, I believed that a task could be completed in 1 ideal day, and the team’s velocity was 2.5 ideal days per week (0.5 ideal days per day), then I could expect the task to take 2 days to complete. But I found this conversion between ideal days and real days confusing. The technique also had the unwanted side-effect of encouraging me to think in absolute time instead of relative time, so was really no better than estimating in real days.

The use of *T-shirt sizes* has a certain appeal, in that it provides a mechanism for quickly conveying high-level information from developers to managers. In my experience the drawback is that, being non-numeric, these units cannot be used to perform arithmetic, so cannot directly be used to predict schedule.

I prefer the use of abstract *points* for relative estimation. Point values can be used to express the relative time it will take to complete a unit of work, compared to other units of work. Early on I was encouraged to think of points as representing the “size” of a unit of work or the “effort” required to complete it. Those terms, although somewhat helpful, never seemed to describe what it was I was actually estimating, and I eventually came to think of estimating in points as estimating in terms of *relative time*.

You can read more on this subject in the article titled What Are We Estimating, Anyway?)

*Read more about estimation in the 99-cent Kindle pocket guide, *Practical Estimation.