There are several reasons that baseball is unique. It’s the only major sport where the defense has the ball to begin every play. The game doesn’t end because time ran out. The playing field is not uniform.

This is the first of a two-part series. Initially, the foundation for park factors will be laid out. Next week, a survey of all 30 MLB yards will be provided.

Park factors are designed to flesh out biases, so the effect is solely due to the ballpark. They’re expressed as three-year averages since one season (or even two) is not sufficient to level all forms of variance.

Comparing performance home and away mitigates the quality of the hitting and pitching. That is, strong offenses will produce more at home and on the road while weak lineups still struggle wherever they play. Similarly, a solid pitching staff will excel home and away while lesser hurlers will struggle, regardless of venue.

To demonstrate this, here is the basic park factor formula for runs.

Runs Index = ((Runs scored at home + Runs Allowed at home)/(No. Home Games)) / ((Runs scored away + Runs Allowed away)/(No. Away Games))

Literally anything can be used in place of runs. Depending on the metric, games could be replaced by at bats, or innings pitched.

Some of the more common factors are home runs, strikeouts and walks. Factors for doubles and triples are incorporated into the projections available in the Fantasy Index Baseball Draft Kit.

Distance and height of the outfield fences are obvious determinants. The amount and location of four territory is another “on the field” constituent of park factors. One of the reasons the Oakland Coliseum is so pitcher friendly is the vast acreage of foul territory. Conversely, the limited foul area in Fenway Park adds to its hitting allure. Curiously, Citizens Bank Park in Philadelphia is at or near the top of the league in foul outs. In this instance, it isn’t the amount of foul territory, but rather layout, where there is an inordinate level of foul ground where foul pops land.

Another major factor is the batter’s eye. This is the area beyond the center field fence, directly behind the pitcher. Some batter’s eyes make it easier or harder to pick up the ball out of the pitcher’s hand.

Everything cited so far related to the physical dimensions and layout of the ballpark. Where the park is located has a huge impact on how it plays. The three major factors are temperature, altitude and wind. All three influence the baseball’s flight. This can be after it is hit, along with the movement out of the pitcher’s hand.

Temperature and humidity go hand in hand. A common misperception is humid conditions hinder ball flight. Humidity is water and the thought is water is heavier than air. However, the molecules water vapor displace are heavier than gaseous water, rendering the air less dense. Lower density results in less resistance, hence increased travel. Warm air can hold more water vapor than cooler air, so baseballs travel further in warmer temperatures.

Higher altitude renders thinner air, which is the same as less dense. The ball travels further at altitude, which of course is why Coors Field is a hitter’s paradise. Here are the top five venues in terms of altitude:

1. Coors Field (Denver, Colorado)
2. Chase Field (Phoenix, Arizona)
3. Turner Field (Atlanta, Georgia)
4. Kauffman Stadium (Kansas City, Missouri)
5. Target Field (Minneapolis, Minnesota)

Altitude also affects the movement of pitches. Curve balls and sliders don’t break as much since the spin needs more air resistance to be more effective. Four-seam fastballs are an optical illusion with the batter thinking they’re going to drop more than they do. The overspin forms a cushion under the ball, preventing it from dropping as much. Thinner air generates less cushion, so it drops more, and doesn’t fool the hitter.

Wind is clearly a factor. A recent study was published by Mike Petriello on mlb.com using wind data generated by Statcast. Those not intimidated by math should check out Wind effect: These ballparks, players were most impacted by the weather.

With the caveat only two years of data went into the study, one of the most interesting takeaways was wind took away more homers in Kauffman Stadium (Royals) than any other venue. Wrigley Field (Cubs) was next, followed by T-Mobile Park (Mariners) and Citizens Bank Park (Phillies).

Wind aided the most homers in Citi Field (Mets). Next were The Oakland Coliseum (Oakland) and Fenway Park (Red Sox). These three parks may seem nonintuitive since they all suppress long balls, but it makes sense that wind would help more since fly balls need more help to clear the fences (or Green Monster) in these venues. Overall, wind prevents far more homers than it generates.

Circling back to the limited two-year sample, there isn’t enough data to definitively classify how wind influences each venue. For now, it’s assumed that wind effect is backed into the respective park factors. Once ample data is collected and studied, factors could be refined.

As mentioned earlier, park indices extend beyond runs and homers. Two of the most overlooked examples are strikeouts and walks. Foul territory is integral to the effect. More foul outs render fewer chances to walk or punch out. Plus, less foul territory generates more strikes. The batter’s eye influences strikeouts and walks. Spin is easier to pick up in some venues. Altitude matters since some pitches are less effective in thinner air. When a pitcher moves to a new venue, it’s not a surprise that their projected ERA and WHIP change. However, some fantasy managers are surprised when the projected strikeout and walk totals are different.

Another common misperception is home parks also embellish scoring. How a park plays with respect to runs combines all the factors. There are some venues that promote homers but restrict runs. In these instances, the fences are generally closer, which means there is less acreage for the outfielders to defend. The result is more fly ball outs than in a bigger venue. The opposite is also true in that a big outfield pasture may hinder home runs, but fuel more fly ball hits, often doubles and triples.

This is a good place to stop. Next week, the factors for every MLB park will be reviewed and explained.

Todd Zola is an award-winning fantasy baseball writer and 2020 inductee into the Fantasy Sports Writers Hall of Fame. He's the content provider for the 2025 Fantasy Baseball Index Draft Kit and the Editor-in-Chief for the 2025 relaunch of the Fantasy Baseball Index magazine.