A park factor compares runs scored in a ballpark with what the same league produced elsewhere. BullpenEdge uses up to six seasons of games, weighting recent seasons more, and regresses every park toward neutral by the equivalent of 90 average games — a single season at a park is far too noisy on its own. Parks with longer history also draw on Retrosheet game logs back to 2010.
The biggest run parks
- Coors Field (Rockies): +21.6%. Altitude thins the air, balls carry, breaking balls break less, and the huge outfield leaves room for hits.
- Sutter Health Park (Athletics, Sacramento): +12.5% so far — a minor-league park with short power alleys and hot summer nights. Only two seasons of data, so it is still heavily regressed.
- Nationals Park: +6.6%.
- Chase Field: +5.3%.
- Fenway Park: +4.8%, mostly doubles off and over the Green Monster.
- Great American Ball Park: +3.9%.
The best pitcher's parks
- T-Mobile Park (Mariners): −9.9%. Heavy marine air at night suppresses carry.
- Petco Park (Padres): −7.9%.
- Progressive Field (Guardians): −5.8%.
- Tropicana Field (Rays): −5.2%.
- Oracle Park (Giants): −4.7%, with cold wind off the bay and a deep right-center field.
Why a park factor is not the whole story
The factor is an average over a season. A 50°F April night at Wrigley with the wind blowing in plays nothing like a 90°F July afternoon with it blowing out, which is why weather is modelled separately (see How weather affects MLB scoring). Roof status matters for retractable-roof parks too. Every game page shows its park's factor next to the forecast, and Run Environment ranks today's slate.
Playing at home is worth something on its own, separate from the park itself: MLB home-field advantage measures how much.