Measuring ISO

Baseball, MLB article at Knup Sports

What is ISO?

In baseball, Isolated Power or ISO is a sabermetric computation used to measure a batter’s raw power. One formula is slugging percentage minus batting average. The final result measures how many extra bases a player averages per at bat. A player who hits only singles would thus have an ISO of zero.

Why do we need this ISO measurement?

Batting average doesn’t tell us everything such as how many of the hits were for extra bases. Slugging percentage does no distinguish between singles and extra base hits. ISO shows which hitters have extra-base power.

More baseball reading at Baseball Spotlight

Some historical background of ISO

Branch Rickey played around with this stat in 1954 when he wrote an article that appeared in Life magazine in which he discussed a formula  that stress the importance of a team scoring more runs than it allowed over the entirety of a season.

Excellent      .250

Great             .200

Above Ave     .170

Average          .140

Below Ave      .120

Poor                 .100

Awful                080

ISO Looking Back

Using the measure of 3000 plate appearances only three players in baseball history have a career ISO of better than .300

  • Babe Ruth .348
  • Mark McGwire .325
  • Barry Bonds   .309

In 2001, Bonds has the highest single ISO ever recorded at .536.


ISO is not a good indicator for future power of a player.

More Sabermetric articles:

  1. Runs Created (RC)
  2. OPS
  3. SLG%
  4. OBP and wOBA


About the author– Tom Knuppel has been writing about baseball and sports for a few decades. As an avid St. Louis Cardinals fan he began with the blog CardinalsGM. Tom is a member of the United Cardinals Bloggers and the Baseball Bloggers Alliance. He also maintains the History of Cardinals website. More recently he has been busy at KnupSolutions and the primary writer of many sports at KnupSports and adds content at Sports 2.0. Tom is a retired High School English and Speech teacher and has completed over one hundred sportsbook reviews. He also can be followed on Twitter at tknup.

Feel free to contact Tom at

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