The Costco Park Effect: Great for Groceries, Not So Great for October Offense?

I love Costco. The air-chilled chicken? Fantastic. The artisan rolls? A staple in the Basewinner cookbook.

But when it comes to baseball, I'm not nearly as enthusiastic about the warehouse experience. 🛒⚾

With Milwaukee and Tampa Bay hosting the League Championship Series, I got curious about how teams perform offensively in what I affectionately call "Costco Parks."

You know the places. Big enclosed buildings, artificial lighting, climate-controlled conditions, and a roof over your head. Not exactly a beautiful October sunset at Chavez Ravine.

So we fired up the Basewinner database and did a little digging.

What Qualifies as a Costco Park?

For this study, we identified six ballparks:

• Chase Field — Arizona

• Daikin Park — Houston

• loanDepot park — Miami

• American Family Field — Milwaukee

• Tropicana Field — Tampa Bay

• Globe Life Field — Texas

We classified games as Costco games only when the roof was recorded as closed or the stadium was listed as a dome.

Using 2026 regular-season data, we compared offensive production in these environments against games played elsewhere.

Our primary measure is BaseRuns per nine innings (BR/9), which estimates offensive production from a team's underlying batting statistics rather than simply counting actual runs scored.

And unfortunately, the results aren't particularly friendly to our Dodgers and Guardians positions.

Milwaukee: Does the Roof Matter?

Milwaukee generated 13.9% more estimated runs per nine innings in home games when the roof wasn't recorded as closed, compared with its roof-closed games.

That's a pretty interesting difference considering we're talking about the same ballpark.

Even more interesting, the Brewers actually struck out less with the roof closed. Their decline in production appears more connected to power than contact.

The isolated power difference, .150 versus .172, certainly caught my attention.

Of course, these aren't identical opponent or pitching samples, and we haven't independently verified that every game not recorded as closed was played with the roof open.

Cleveland: The Biggest Costco Concern

Woof.

Cleveland generated 46.5% more estimated runs per nine innings in other road parks than in Costco Parks, using its Costco production as the baseline.

But the statistic that really jumps off the page is the strikeout rate.

The Guardians struck out in 27.27% of their Costco Park plate appearances compared with just 19.54% in other road environments.

That's a difference of nearly eight percentage points.

They also walked less frequently and generated less power.

Now, 495 plate appearances isn't a huge sample, and the quality of opposing pitching could certainly be contributing to these results.

Still, with the Guardians heading into an enclosed environment for the ALCS, it's a split worth knowing.

Los Angeles: Even the Dodgers Aren't Immune

The Dodgers generated 16.6% more estimated runs per nine innings in other road parks compared with Costco Parks.

What's interesting here is that their plate discipline actually improved in the enclosed environments.

Their strikeout rate declined, and their walk rate increased.

But the power production wasn't nearly as impressive.

ISO dropped from .171 to .145, while home runs per plate appearance fell from 3.35% to 2.39%.

That's worth considering with Los Angeles playing games at American Family Field.

What Does This Mean for Our LCS Positions?

We've established positions on both Los Angeles and Cleveland, so naturally I'd prefer these numbers pointed in the opposite direction.

But there's much more to handicapping these series than historical performance in a particular ballpark environment.

Our Basewinner models incorporate much more matchup-specific information, including starting pitching, bullpen strength, offensive ratings, lineup construction and the adjustments we've made for postseason pitching rotations.

Those factors carry considerably more weight in our projections than these relatively small samples.

It's also important to distinguish a true roof effect from a ballpark effect. The six Costco Parks have different dimensions, surfaces and offensive characteristics. Opponent quality and pitching matchups could explain some or even much of the differences we're observing.

And for totals bettors, lower offensive production in one environment doesn't automatically mean an under. We still have to evaluate both offenses, the pitching matchups and the specific market price.

The Basewinner takeaway: I still like our Dodgers and Guardians positions. The Costco Park splits are simply another interesting piece of the October handicapping puzzle, particularly when evaluating game totals and team totals.

I'll happily take Costco's air-chilled chicken and artisan rolls home.

I'm just not convinced I want my October baseball played inside a warehouse.

Much love...BW

Methodology: Basewinner analysis of 2026 MLB regular-season game and player boxscore data. Costco Parks include Arizona, Houston, Miami, Milwaukee, Tampa Bay and Texas when games were recorded as Roof Closed or Dome. BaseRuns/9 uses a simplified BaseRuns calculation normalized to 37.5 plate appearances. Percentage increases use Costco or roof-closed production as the baseline. Results are descriptive, not adjusted for opposing pitching or individual park effects.