What Are Park Factors in Baseball?

A .300 average in one ballpark isn’t worth the same as a .300 average in another. Baseball is the rare sport where the field itself changes size and shape from city to city, and those differences move the numbers enough to fool you if you don’t account for them. That’s what park factors are for.

From the scorebook — A park factor of 100 is neutral. Coors Field routinely grades well above that as the most run-inflating park in baseball, while pitcher’s parks sit below 100. The same swing can be a home run in Denver and a warning-track out by the bay.

What a park factor actually is

A park factor is a single number that captures how much a ballpark boosts or suppresses offense relative to a neutral one. The scale is centered on 100: a factor above 100 means the park helps hitters, below 100 means it helps pitchers. It’s built by comparing how many runs (or home runs, or doubles) are scored in a team’s home park versus its road parks, so it isolates the ballpark from the quality of the teams playing in it. Once you have it, you can strip the ballpark out of a player’s stats and compare a Rockies hitter to a Padres hitter on fair terms.

Coors Field: the altitude problem

The most extreme park in the game is Coors Field in Denver, and the reason is physics. The stadium sits roughly 5,280 feet above sea level — a full mile up — and the thin mountain air does two things at once: it lets batted balls carry noticeably farther, and it flattens the movement on breaking pitches so they’re easier to hit. To fight the home-run barrage, the Rockies built an enormous outfield, which then created vast gaps for doubles and triples. The result is historically the most run-inflating environment in MLB, and it produces some of the largest home-road splits you’ll ever see, feeding the long-running “Coors hangover” theory about hitters struggling on the road.

Fenway Park: the Green Monster

Fenway Park in Boston warps offense in a different, more localized way. Its left-field wall — the Green Monster — stands about 37 feet tall and sits only around 310 feet from home plate. Fly balls that would settle into a left fielder’s glove almost anywhere else clang off the wall for singles and doubles instead. Fenway is a doubles machine as a result, and it especially rewards right-handed hitters who can pull the ball into the Monster. It’s a reminder that a park factor isn’t only about total runs — it reshapes which kinds of hits happen.

The other end: pitcher’s parks

Not every park helps hitters. Spacious parks with deep dimensions and heavy night air — like the marine-layer conditions along the San Francisco and San Diego coasts — suppress offense, grading below 100. A pitcher who posts a solid ERA in a run-friendly park may actually be having a better year than one with a shinier number in a pitcher’s haven, once you adjust.

Why it matters for reading stats

Park factors are the reason smart analysis leans on adjusted metrics. Stats like OPS+ for hitters and ERA+ for pitchers are set to a scale where 100 is league average after correcting for the ballpark, so Pedro Martinez’s 2000 season or a slugger’s home-run total can be compared across wildly different environments. It’s also the same idea baked into the run-environment modifiers in 162-0: the game changes with the era and setting, so a raw stat line never tells the whole story. For more of the vocabulary, see the 162-0 glossary, and for how the single number of WAR folds all of this together, read what WAR means in baseball.