Calculating Baseball Stats - Isolated Power (ISO)

If you're a baseball fan, you probably are aware of how important statistics are to the game. A casual fan probably understands what most of the most popular statistics measure, but what about some of the more advanced statistics created to measure a players worth? In this article, we'll look at a one of the slightly more advanced statistics, what it means, and how it's calculated. If you do not already have a familiarity with how to calculate batting average, on base percentage, slugging percentage, and total bases, you may want to see my articles on those topics before diving into this material.

Isolated Power, or ISO, is a statistic that was created invented by the famous general manager Branch Rickey and Al Roth in the 1950's (this pair also invented On Base Percentage). Isolated Power attempts to separate and measure a player's ability to hit for power as exhibited in extra base hits (doubles, triples, and home runs). Where batting average counts all hits equally, and slugging percentage gives credit for total bases, but also counts singles, ISO measures just the player's extra-base abilities. Isolated power is basically a ratio of Extra Base Hits per at bat.

To calculate Isolated Power, you must first determine the Total Bases a player has accumulated. This is done with the following formula:

Singles (Total hits - 2b - 3b - HR) + (2b x 2) + (3b x 3) + (HR x 4) = Total Bases

Once you have determined a players Total Base total, you're ready to calculate their Isolated Power. To calculate Isolated Power, subtract the players hit total from their Total Bases, then divide by the number of at bats the player has.

So, let's look at a real world example or two. In 2005, Alex Rodriguez of the New York Yankees led the American League in Slugging Percentage (.610) while hitting .321. But what was his ISO? Rodriguez had 369 Total Bases in 2005, and 194 hits in 604 at bats. So, when we calculate ISO for Rodriguez, the formula looks like this:

369 Total Bases - 194 Hits / 605 At Bats = .289 ISO

Alex Rodriguez won the 2005 American League MVP (Most Valuable Player) award. The runner up, David Ortiz, finished the season with 363 Total Bases on 180 hits in 601 at bats. That gives him an Isolated Power of .304, slightly higher than Rodriguez'. This does not mean that Ortiz had a better year than Rodriguez, simply that he exhibited more "raw power".

Another way to calculate Isolated Power is take a players slugging percentage, and subtract his batting average. For example, Rodriguez' 2005 slugging percentage was .610, and his batting average was .321. The difference is .289, the same number we came up with above. This works because slugging percentage measures a players extra base hit ratio plus his hits per at bat ratio, and batting average measures the ratio of hits per at bats. When you subtract batting average from slugging percentage, you are left with the ratio of extra base hits per at bat, which is Isolated Power.