Milwaukee's Brewers complete a 3.7% chance sweep, and favorites keep losing
Week 5 had baseball's league championship series, the NFL's sixth weekend, a Champions League matchday, and the first two games of the WNBA Finals. Kalshi's game-winner markets in the sports we track traded $1.79B, up 1.4% from Week 4. Polymarket's international exchange traded $728M across all sports markets, up 13.8%, and most of that gain came from soccer.
The Brewers swept a series the game markets gave them a 3.7% chance of sweeping, primetime games took half of Kalshi's NFL money, and favorites lost more often than their prices said. This is how prediction markets traded major sports in the past fortnight.
After a short week-long hiatus from our weekly sports newsletter, we are back. We skipped the newsletter last week to perfect and launch our Q3 2026 Sector Report. It delved deeper into prediction market activity in Q3 2026 and offered an analytical view of sports we haven’t done before. One key finding was that tennis led sports at $9.17B in notional volume, and baseball came next at $7.48B despite the World Cup final falling within the quarter. If you missed it, the September preview is a quick way in.
Because we skipped a week, this edition covers two: a short catch-up on Week 4 (Oct. 6 to 12) and the full Week 5 (Oct. 13 to 19).
Week 4 in brief
Week 4 was the busiest week of the season by several counts. DeFi Rate recorded $19.03B traded on Kalshi across all categories in the week to Oct. 11, the highest in its six-week series, while combo contracts made up 51.5% of Polymarket US trade volume and 60.2% on Sunday, Oct. 11. In the sports we track, Kalshi traded $1.76B and Polymarket $640M.
Bills at Rams on Monday night drew $104.4M on Kalshi, the biggest NFL game of the two weeks. Max Verstappen won his first Singapore Grand Prix on $1.44M of Kalshi race-winner trading, a quieter race for traders than the one at Sepang we wrote up in Sir Lewis Hamilton's Impossible 96% Swing That Almost Nobody Acted On.
Our Week 4 calls went three for five. Soccer missed at $183.9M against a $190M bar, and so did the baseball call, while the NFL, NHL, and NBA calls all hit. The running record is six hits from 11.
Week 5 in Numbers
Week 5 had baseball's league championship series, the NFL's sixth weekend, a Champions League matchday and the first two games of the WNBA Finals. Kalshi's game-winner markets in the sports we track traded $1.79B, up 1.4% on Week 4. Polymarket's international exchange traded $728M across all sports markets, up 13.8%, and most of that gain came from soccer.
Milwaukee swept a series the game markets gave 3.7%
The story of the week is Milwaukee. The Brewers swept the two-time champion Dodgers even though the game markets made them the underdog in three of the four games. Multiply those four prices together and a sweep had a 3.7% chance. For anyone pricing related contracts, the more useful finding is that Polymarket's pennant market tracked the single-game markets the whole way. Between games, it never sat more than about four points from what the game prices implied.
Milwaukee beat the Dodgers 2-1, 7-4, 6-2 and 6-4 between Oct. 11 and 15 and reached its first World Series since 1982. Going into each game, Polymarket's moneyline priced the Brewers at 50.5¢, 45.5¢, 38.5¢ and 41.5¢. They were the underdog three times out of four. They won all four. Multiply the four prices, and a sweep had a 3.7% chance.
The more useful question for anyone running a book across related contracts is whether the series market agreed with the game markets. We took the Brewers' average game price of 44¢ and worked out the chance of winning a best-of-seven from each scoreline. Before Game 1 that gave 37.1¢, against a pennant price near 40¢. At 1-0 it gave 53.8¢ against 53.7¢. At 2-0 it was 72.9¢ against 70¢, and at 3-0, 90.2¢ against 90.2¢.
Two order books, run by different traders, landed on the same number after every game, and that kind of internal consistency is what an allocator wants to see before trusting a market's prices on anything bigger than a single game.
Game 3 traded $34.3M on Kalshi's winner markets, the biggest baseball game of the fortnight, while on Polymarket the same game drew $3.39M, missing the $3.40M bar in our Week 4 call by $5,013.
Where the money sits this week
Week 5 sports volume held level on Kalshi at $1.79B, up 1.4%, and rose 13.8% on Polymarket to $728M. Soccer did the lifting. A Champions League matchday and a full weekend in Europe's top leagues took Polymarket soccer to $295M and Kalshi soccer to $288M, the only sport where the two exchanges trade about the same amount. College football came within $4.6M of the NFL on Kalshi, at $443M against $447M.
The NFL money bunches up. Over the last two weekends, the six primetime games carried 50.8% of Kalshi's NFL winner dollars while making up 6 of 29 games. A Monday night game averaged $91.6M, 6.5 times an early Sunday game, and Kalshi out-traded Polymarket in all 29 games by a median of 6.3 times. For an institution sizing a position, depth is a scheduling question as much as an exchange question.
This week’s favorites were expensive
Across 1,219 games on Polymarket from Oct. 6 to 19, favorites were priced at 65.8% on average at the start and won 59.9% of the time. The shortfall showed up in six of eight sports: 10.5 points in UFC, 8.4 in college football, and 7.3 in tennis. Hockey went the other way. NHL favorites won 64.9% of games at an average price of 59.1¢. Two weeks is a small sample, and we will keep tracking it, but 110 losses from 393 favorites priced at 70¢ or more is a lot of expensive paper.
The Cardinals were the loudest example of all. They started at 12.5¢ at the Rams, climbed to 81¢, fell back to 23¢ late, and won. Earlier this season we traced a similar ride in the 12¢ comeback in Rio, when the Ravens went from about 12¢ to 99¢ in the final minutes. In esports, FURIA knocked Cloud9 out of the League of Legends Worlds play-in from 17.5¢, and in Wuhan, Ann Li beat Aryna Sabalenka from 16.5¢.
What we are watching in Week 6
Five calls for Oct. 20 to 26. Each one is about market activity, not results.
World Series Game 1 tops $34.3M on Kalshi's winner markets, the NLCS Game 3 figure.
Kalshi NBA winner markets average at least $4.3M a game in opening week, double the preseason's $2.14M.
NFL Week 7's three primetime games are Kalshi's three biggest NFL games, as they were in each of the last two weeks.
Kalshi's US Grand Prix winner market tops Singapore's $1.44M.
Polymarket esports tops $150M as the League of Legends Worlds Swiss stage opens.
Prediction Frontier publishes data for information only; nothing here is investment, trading or betting advice.