Prediction market activity rebounded this week after three consecutive weeks of declines. Combined comparable trading volume on Kalshi and Polymarket Global increased 5.8% from the previous week, reaching $10.30 billion.
Kalshi remained the clear market leader, accounting for 81.3% of combined comparable volume, while Polymarket Global posted its first weekly increase in a month.
One trend stood out: comparable trading volume increased even as the number of recorded transactions declined. Average notional per transaction rose 8.4%, suggesting that larger trades drove much of the week’s rebound.
Political trading also jumped sharply on US-based platforms, while economic trading on Polymarket Global dropped by more than 90%.
| Metric | Kalshi | Polymarket Global | Polymarket US | Combined / Share |
|---|---|---|---|---|
| Comparable Notional Volume (USD) | $8,382,009,522 | $1,921,736,483 | Unavailable | $10,303,746,005 |
| WoW | +6.2% | +4.1% | N/A | +5.8% |
| Native Headline Volume | 8.07B displayed contracts | $1.92B USD notional | 846.2M DMR contracts | Non-additive |
| Native Volume WoW | -4.1% | +4.1% | -2.4% | N/A |
| Transactions | 56,509,439 | 19,087,508 | N/A | 75,596,947 |
| Open Interest (Snapshot) | $728,977,722 | $343,638,226 | 31,372,787 contracts | Non-additive |
| Active Markets | 76,756 | 133,598 | N/A | Non-additive |
Metric note: The native headline metrics are not additive. Kalshi’s displayed contract volume, Polymarket Global’s USD notional and Polymarket US’s official DMR contract volume use different venue-native semantics. Category volume comparisons below use a separate DefiRate USD-notional decomposition across the three platforms.
Kalshi share of comparable notional volume
Kalshi as a percentage of combined Kalshi and Polymarket comparable USD notional, by week ending.
Kalshi held between 72% and 76% of the two-venue total for four straight weeks, then stepped up to 81.0% in the week ending Aug 2 and held 81.4% through Aug 9. The move came from Polymarket contracting 38.8% in the Aug 2 week rather than from Kalshi expanding.
Source: DeFi Rate aggregation, Kalshi Trade API and Polymarket on-chain activity · defirate.com/prediction-markets/
US political trading surges ahead of the next round of primaries
Politics was one of the biggest stories of the week. Total political trading across the three platforms reached about $105.7 million.
But the platforms moved in very different directions:
- Kalshi: $33.7 million, up 118.9%
- Polymarket US: $17.9 million, up 81.0%
- Polymarket Global: $54.0 million, down 26.8%
On Kalshi, the increase was led by Michigan’s Democratic Senate primary, where Haley Stevens and Abdul El-Sayed contracts generated about $7.5 million combined. Polymarket US activity was more mixed: Florida Republican governor-primary contracts accounted for about $5.9 million, while the Clacton by-election and Michigan Senate margin markets were also major contributors.
Economic prediction markets generated about $55.2 million during the week. Kalshi performed well in this category:
- Kalshi: $47.3 million, up 10.6%
- Polymarket Global: $5.2 million, down 91.2%
- Polymarket US: $2.7 million, down 69.0%
Much of the huge drop on Polymarket Global reflects the prior week’s roughly $50.1 million of activity in the July Federal Reserve decision market, which fell out after the meeting was resolved. Once that event was over, much of the trading activity disappeared.
Sports remains dominant as NFL trading begins to accelerate
Sports continued to be the biggest source of trading activity across all three sites. Across Kalshi, Polymarket Global and Polymarket US, sports-related markets generated about $2.64 billion in the category-level USD-notional decomposition.
- Kalshi: $2.01 billion, down 9.2%
- Polymarket US: $329.3 million, down 27.6%
- Polymarket Global: $299.2 million, down 1.2%
Kalshi accounted for roughly 76% of sports trading volume across the three platforms on that basis.
Esports and Middle East markets drive Polymarket Global activity
Polymarket Global had a particularly interesting mix of markets this week.
Markets related to the Strait of Hormuz and tensions involving Iran attracted significant attention. The market asking whether Strait of Hormuz traffic would return to normal by Aug. 31 saw its trading volume increase 119.1%, reaching $5.37 million.
A shorter-term version of the same question, with an Aug. 15 deadline, increased 398.4% to $3.84 million. Another market asking whether the US would invade Iran before 2027 generated $3.54 million in volume.
Another major source of activity was esports.
Six of Polymarket Global’s 10 biggest individual markets were esports matches.
A Dota 2 match between 1win and BetBoom Team generated $4.98 million in trading volume. Several League of Legends matches generated more than $18 million combined.
Baseball leads as NFL game markets begin to take over
The US platforms were heavily focused on sports, especially baseball. Kalshi’s biggest markets included individual MLB games.
For example:
- White Sox vs. Red Sox: $18.73 million in volume
- Boston vs. Los Angeles Dodgers: $12.74 million
- Several tennis markets also generated more than $23.8 million combined.
The Carolina-Arizona NFL preseason winner market generated $11.52 million; Kalshi’s broader public game-page volume across every direct market attached to the matchup reached 21.18 million displayed contracts.
Polymarket US was even more concentrated on baseball. All 10 of its biggest markets were MLB game-winner markets. The White Sox vs. Red Sox market had 8.51 million contracts traded, while Dodgers vs. Diamondbacks had 6.09 million.
What the trading data tells us
Combined comparable notional volume on Kalshi and Polymarket Global increased 5.8%, but the number of recorded transactions fell 2.4%. Average notional per recorded transaction therefore increased about 8.4%, from $125.78 to $136.30. That means the increase in overall comparable notional was associated with higher average transaction value, not a larger transaction count.
Average notional per recorded transaction
Weekly comparable notional divided by recorded transactions, in US dollars, for each venue and the two-venue combined series.
Average transaction value fell for four consecutive weeks, from $175.09 in the week ending Jul 5 to $125.78 in the week ending Aug 2, then rose 8.4% to $136.30. Both venues moved the same direction in the final week: Kalshi from $138.29 to $148.33 and Polymarket from $90.71 to $100.68. Polymarket’s average has recovered roughly a fifth of its four-week decline; Kalshi’s has recovered about a third.
Source: DeFi Rate aggregation, Kalshi Trade API and Polymarket on-chain activity · defirate.com/prediction-markets/
Polymarket Global alone saw its number of recorded transactions fall 6.2% to 19.1 million.
Polymarket US’s open positions fell from 62.88 million contracts to 31.37 million, a 50.1% decline. Expiring and settled contracts likely contributed, with open positions closing faster than new open interest accumulated during the week. The snapshot data does not establish the full cause.
Polymarket Global had 133,598 active market records, compared with 76,756 on Kalshi.
The saved listing count shows more active market records on Polymarket Global, although differences in contract structure and counting methodology limit a direct breadth comparison between the two platforms.
Polymarket US volume against open positions
Official Daily Market Report trade volume against the week-close open interest snapshot, both in millions of contracts.
Trade volume fell 36.4% across the six weeks and has been close to flat since the week ending Jul 26, moving from 869.5 million to 846.2 million contracts. Open positions fell 66.6% over the same period, from 94.1 million to 31.4 million contracts, with more than half the decline landing in the final week. Weekly turnover therefore ran at roughly 27 times the closing open interest by Aug 9, against 14 times in early July.
Source: Official Polymarket US Daily Market Reports · defirate.com/prediction-markets/
What to watch this week
As the NFL preseason continues, early signs suggest activity is building much faster than it did a year ago. Kalshi’s direct game-winner basket had already reached roughly 394,000 contracts by Aug. 4, about 7.2 times the comparable point last season.
The Carolina-Arizona opener then showed what happens when live-game boards replace preseason futures as the center of activity. Kalshi’s broader game winner market reached $21.18 million, while Polymarket Global added about $409,000 in event-level USDC notional and Polymarket US reported 3.40 million contracts.
Growth through the NFL season will not look the same on every platform. About 74% of tracked Kalshi NFL volume was already tied to game markets, compared with only about 4% on Polymarket Global, where roughly 96% remained in futures. That points to a likely next phase in which Kalshi and Polymarket US accelerate as more live matchup markets open, while Polymarket Global may remain more futures-driven until game liquidity deepens.
The next question is whether last week’s US-platform surge was a one-event spike or the start of a broader primary-season build. Michigan was an important driver, but the pipeline is not empty: upcoming Wisconsin, Minnesota and South Carolina contests on Aug. 11 and Florida primaries on Aug. 18 give traders another sequence of scheduled events.
The clearest sign of sustained growth would be volume rotating quickly from resolved Michigan markets into those upcoming contests. If that happens, political volume could remain elevated; if not, last week’s jump may prove temporary.
