MLB Park Factors

Baseball is the only major American sport where the playing field's dimensions vary, and the variation is large enough to change a total by more than a run. Park factors are the most stable, most knowable input in baseball handicapping — which also means they are thoroughly priced in.

What A Park Factor Is

A park factor compares scoring in a venue with scoring in an average venue, controlling as far as possible for the teams playing there. An index of 110 means roughly 10% more scoring than average; 90 means roughly 10% less.

The main drivers:

Altitude. Thin air reduces drag, so balls travel further and breaking pitches move less. This is the largest single effect in baseball and it is why one venue sits far outside the range of all the others.

Outfield dimensions and wall heights. Distance to the fences and how high they are, which affects home runs and doubles differently.

Foul territory. Large foul areas convert would-be foul balls into outs, suppressing offence in a way that is invisible on a highlight reel.

Prevailing wind and orientation. Some venues have consistent wind patterns that push or hold fly balls depending on direction.

Roof status. Retractable roofs create two different parks in the same building.

The Range, And Why It Is Not An Edge By Itself

Every book knows these numbers. A total in a hitter's park is already elevated, so betting the over because the park is friendly is betting on information the price contains.

Interaction With Pitcher Profile

Park effects are not uniform across pitchers, and this is where the useful work is.

Fly ball pitchers are punished far more than average in home-run parks and helped far more in spacious ones. A ground ball pitcher is comparatively park-neutral.

Breaking-ball pitchers lose more at altitude than pitchers who rely on fastballs and changeups, because reduced air resistance flattens curve and slider movement.

Left-handed and right-handed hitters are affected differently by asymmetric parks, so a lineup's handedness composition interacts with the venue.

A generic park adjustment applied to a pitcher whose profile is unusually sensitive to that park is a place where the market's blanket adjustment can be wrong. See starting pitchers.

Weather On Top Of Park

Temperature. Warm air is less dense, so balls carry further. The effect is meaningful across the range from a cold April evening to a hot August afternoon — worth several tenths of a run.

Wind direction. In open parks, wind blowing out is worth a substantial amount and wind blowing in suppresses scoring. Ten to fifteen mph out can be worth half a run or more in the right venue.

Humidity. Counterintuitively, humid air is slightly less dense than dry air, marginally aiding carry. The effect is small.

Day-to-day weather is where park analysis becomes actionable, because the park is constant and priced while the day's conditions are variable and sometimes not fully absorbed — especially for early-afternoon games where the forecast changed overnight. See MLB totals.

Frequently Asked Questions