Leaders—good leaders, that is—put a lot of thought into the makeup of the various teams for which they’re responsible. They’re very concerned with, as Jim Collins put it, “getting the right people on the bus.” I’ve even written about the 5 people you should have on your team (you can read that post by clicking here). But, what about the size of the team? It turns out, when it comes to teams, size does matter.

Like you, I’ve been involved in a lot of teams. Work teams, sports teams, social teams, mastermind teams, and so on. So we all have experiential and anecdotal evidence of optimal team sizes. But what does the science say? What really is the best size for your teams?

In their terrific book, Team Genius: The New Science of High-Performing Organizations, authors Rich Karlgaard and Michael S. Malone present three so-called “Magic Numbers” that can help you determine the best size for your small, medium, and large teams.

Magic Number #1: 7 (plus or minus 2)

You’ve probably heard this one, and with good reason. Beyond this number (and certainly beyond 12, which is the maximum functional size for a small team), the team will often devolve into groups of two or three. Apparently this has something to do with our short-term memory, which in most cases is only capable of holding five to nine pieces of information at one time. Certainly my own experience has convinced me that the optimum number of people for a mastermind group is five. So keep your small teams to 7 (plus or minus 2), and you’ll be in good shape.

Magic Number #2: 150

This is called the “Dunbar number,” after British anthropologist Robin Dunbar, who discovered that the same human group sizes appear over and over, throughout the world. [Side note: Someday, I would like to have a number named after me. I would like that number to be 9.] His research showed that after a medium-sized team (for example, a primitive village, or a religious community) hits a certain number (technically 147.8, but we’ll call it 150), it tends to divide itself into smaller units. This suggests that the maximum size for an effective medium-sized team is 150.

Magic Number #3: 1,500

Hewlett-Packard discovered this one back in the 1950s, when they were one of the most dominant companies in the world. In his autobiography, co-founder David Packard wrote that once the company approached 1,500 employees, “it was increasingly difficult for Bill and me to know everything well and to have a personal knowledge of everything that was going on.” At that point, they realized that the best thing to do was to re-create the company as a “divisional” entity, with each division having no more than 1,500 people.

So, when you’re looking at your own organization and thinking about your own teams, remember these “Magic Numbers”:

Best size for a small team: 7 people (plus or minus 2).

Best size for a medium team: 150 people.

Best size for a large team: 1,500 people.