Isolated Power (ISO) is a measure of a hitter’s raw power. Or, to look at it another way, it measures how good a player is at hitting for extra bases. The simplest way to calculate ISO is to subtract a player’s Batting Average from their Slugging Percentage, which leaves us with a measure of just a player’s extra bases per at bat.

If you prefer, you can also calculate ISO this way:

ISO = ((2B) + (2*3B) + (3*HR)) / AB
ISO = Extra Bases / At-Bats 

It takes a long time for a player’s ISO to have predictive power going forward; a sample size of 550 plate appearances is recommended to draw any conclusions. In other words, if Albert Pujols has a .550 ISO two weeks into the season, it’s way too early to expect that to continue.

Context:

Please note that the following chart is meant as an estimate, and that league-average ISO varies on a year-by-year basis. To see the league-average ISO for every year from 1901 to the present, check the FanGraphs leaderboards.

Links for Further Reading:

Power Factor v. Isolated Power – Walk Like a Sabermetrician

PECOTA’s ISO – Baseball Prospectus