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How to Calculate On-Base Percentage

On-base percentage (also known as on-base average) is the measure of the number of times a player gets on base via hit, walk, or hit by pitch, expressed as a percentage of his total number of plate appearances.

What’s a plate appearance? Unlike an “at bat,” a plate appearance is counted every time a player comes to bat regardless of the outcome of that time at the plate. The statistic known as “at bats” counts only the times a player gets a hit or makes an out, while “plate appearances” count walks, sacrifices, hit by pitch, and so on.

Here’s how you calculate an on-base percentage:

(Hits + Walks + Hit by Pitch) / (At Bats + Walks + Hit by Pitch + Sacrifice Flies)

This stat should have replaced batting average as the basic unit of measure for offensive players because it simply and concisely measures the most important element of being a hitter: getting on base. Batting average is simply not that good at determining how effective an offensive player is.

Why? Because there’s more to getting on base than simply getting a hit. The old saying “a walk is as good as a hit” is almost true. And the problem with batting average is that it completely ignores walks.

A .270 hitter who draws 100 walks per year will probably have an on-base percentage around .400. But a .310 hitter who draws 20 walks languishes down around the .333 mark. Over the course of 600 plate appearances, that amounts to about 35 more times the first guy gets on base compared to the second guy. 35 more times on base would probably equal about 10 or 12 more runs, which might be the difference in winning 4 or 5 games. That can put a team into the playoffs or, conversely, send a team home for the winter.

The bottom line is, the player with the good OBP knows the strike zone and knows how to put himself into position to score a run or drive one in.

What’s considered a good OBP? The league average is usually around .330, and the league leader is usually around .420; good is about .370 or so. Ted Williams, who had the best batting eye in history, is the lifetime OBP leader with a .483 career mark, and until 2002, he also had the highest single-season OBP with .551 when he hit .406 in 1941.

In 2002, Barry Bonds, thanks to 198 walks from scared pitchers and managers, recorded an absolutely astounding OBP of .582. The previous year, Bonds had also broken the long-standing record for slugging percentage. And Bonds was even more efficient at getting on base in 2004 when he posted an astounding .609 on-base percentage — thanks to 232 walks!

View all-time leaders in on-base percentage at Baseball-Reference.com: single-seasoncareer,year-by-year.

Recently, I’ve received questions from readers about why other things aren’t included in on-base percentage, such as reaching base on a fielder’s choice or error. There are two answers: first, it’s impossible to calculate that data after the fact. There’s no statistical record of reaching on error or fielder’s choice, so it would be impossible to assign those numbers to old-time players.

Second, and more importantly, on-base percentage is supposed to measure a player’s ability or skill to get on base. Reaching on a fielder’s choice indicates that you just created an out, but by chance there was someone else on base. Did you help your team? No. Your team is in worse shape as it was one batter ago. Reaching via error is also just a matter of luck. You should have been out, but the fielder screwed up. OBP makes the distinction between getting on base because of your ability and getting on base because of just plain luck.

That’s why, as I started this essay, OBP is possibly the truest measure of a hitter.


The Book of Baseball Literacy: 3rd Edition

For hundreds of descriptions, stats, and stories like this, read The Book of Baseball Literacy: 3rd Edition, now available as an ebook exclusively for Amazon Kindle for just $2.99 and in a print edition for just $14.95. Learn more about the book