- ▸ Kalshi and Polymarket combined for $5.64 billion in weekly notional volume during the week of May 18, while Kalshi rose 3% to $3.99 billion and stayed on pace to challenge its monthly record set in April.
- ▸ Polymarket fell 15% to $1.65 billion, pushing Kalshi’s share of combined volume to 70.8%, up from 66.7% the prior week.
- ▸ Sports remained the largest category on both platforms, but Kalshi’s activity was more concentrated in sports and parlay-style combos, while Polymarket showed a heavier mix of sports, crypto and politics.
Kalshi remained on pace for another monthly volume record during the week of May 18, while Polymarket’s decline widened the gap between the two largest prediction market platforms.
Kalshi posted $3.99 billion in notional volume, up 3% from the prior week, according to DeFi Rate’s volume tracker. Polymarket fell 15% to $1.65 billion, down from $1.94 billion during the week of May 11.
Together, Kalshi and Polymarket generated $5.64 billion in weekly notional volume, down about 3.1% from the prior week but still within the range that has defined the combined market since late April. Kalshi accounted for 70.8% of the total, up from 66.7% the prior week.
With one week left in May, Kalshi has generated about $12 billion across the first three reported weeks of the month, putting it within reach of the $14.81 billion monthly record it set in April. Polymarket has generated about $5.19 billion over the same stretch, leaving it well short of the pace needed to challenge its $10.57 billion March record.
Kalshi vs. Polymarket Volume and Transactions Last 8 Weeks
| Week | Kalshi Volume | Polymarket Volume | Kalshi Tx | Polymarket Tx |
|---|---|---|---|---|
| 3/30/2026 | $2.90B | $1.97B | 20,098,554 | 22,698,487 |
| 4/6/2026 | $3.54B | $2.48B | 22,157,345 | 22,349,643 |
| 4/13/2026 | $3.06B | $2.04B | 20,874,039 | 21,429,287 |
| 4/20/2026 | $3.91B | $1.96B | 23,788,403 | 19,220,738 |
| 4/27/2026 | $3.81B | $1.71B | 23,200,472 | 14,837,848 |
| 5/4/2026 | $4.13B | $1.60B | 24,327,253 | 15,805,153 |
| 5/11/2026 | $3.88B | $1.94B | 24,055,813 | 17,291,328 |
| 5/18/2026 | $3.99B | $1.65B | 25,821,400 | 17,442,879 |
Kalshi holds near $4B as Polymarket pulls back
Kalshi’s latest weekly total kept the CFTC-regulated exchange near the $4 billion level for a third straight week in May. Kalshi reached $4.13 billion during the week of May 4, slipped to $3.88 billion during the week of May 11 and rose back to $3.99 billion during the week of May 18.
Polymarket moved in the opposite direction after a stronger week of May 11. Its 15% decline to $1.65 billion widened Kalshi’s lead in the head-to-head comparison. Polymarket’s volume hasn’t topped $2 billion in weekly notional trading volume since the week of April 13.
The Polymarket US trend was different. Its 7.5% increase to $447.7 million gave the U.S.-facing platform one of the more notable positive signals in the week’s data, even as global Polymarket volume declined.
Transactions moved higher on both of the major platforms. Kalshi processed 25.8 million transactions, up 7.3% week over week, while Polymarket processed 17.4 million, up 0.9%. That means Polymarket’s notional volume fell even as transaction count edged higher, suggesting smaller average trade size compared with the prior week.
What’s driving the momentum
Sports continues to be the largest category across both Kalshi and Polymarket, though the two platforms continued to show different market mixes. Kalshi’s volume was more concentrated in sports and exotics, a category understood to largely capture parlay-style sports markets called “combos,” while Polymarket showed a more balanced mix of sports, crypto and politics.
Kalshi vs. Polymarket Category Comparison (Week of May 18)
| Category | Kalshi | Polymarket |
|---|---|---|
| Sports (incl. Exotics) | $1,599,194,979 | $675,522,559 |
| Crypto | $370,012,038 | $398,990,146 |
| Politics (incl. Trump) | $35,985,918 | $211,744,522 |
| Culture / Entertainment | $5,675,144 | $13,207,989 |
| Economics | $8,357,632 | $17,896,534 |
Top market volumes below are from DeFi Rate’s rolling seven-day prediction markets tracker and reflect USD-at-price, or actual dollars traded, rather than notional volume.
NBA playoff futures a significant driver for Kalshi this week
Kalshi’s top markets were led by sports, with NBA playoff futures and game markets standing out. Its largest market was for the NBA Finals champion, followed by the Los Angeles mayoral election winner, NBA Eastern and Western Conference champion markets and the NHL’s Stanley Cup champion.
The top markets also included individual NBA games, La Liga relegation, the 2026 World Cup winner and the Indian Premier League cricket champion, reflecting a busy late-May sports calendar. The NBA conference finals and NHL playoffs were both active during the week, while soccer and cricket gave Kalshi additional sports depth outside the major U.S. leagues.
Politics appeared in Kalshi’s top markets, but at a smaller scale than on Polymarket. Kalshi’s leading political markets included Los Angeles mayor, the Texas Republican Senate primary, 2028 presidential nominee markets and a new U.S.-Iran nuclear deal market.
Polymarket’s World Cup audience capturing siginificant share
Polymarket’s top markets showed a more global and current-events-driven mix. Its largest market was for the 2026 FIFA World Cup winner, which ranked far ahead of the rest of the platform’s leading markets.
Politics was the other major Polymarket driver. U.S.-Iran peace-deal markets, a Trump speech market tied to events with China’s Xi Jinping, 2028 presidential markets and overseas election markets all ranked among Polymarket’s top markets, giving the platform a much heavier politics mix than Kalshi.
Crypto also remained an important part of Polymarket’s weekly activity. Bitcoin price markets, including when Bitcoin will hit $150,000 and what price Bitcoin will hit in May, ranked among the platform’s leading markets.
Top markets at Polymarket US
Polymarket US continues to lag Kalshi in terms of volume, with $447.7M (+7.5% WoW), as the U.S. app continues to offer mostly sports markets. Its top markets were dominated by NBA, golf, cricket, tennis, WNBA, MLB and NHL contracts.
The largest Polymarket US markets included San Antonio vs. Oklahoma City, CJ Cup Byron Nelson golf markets, cricket match markets and a mix of tennis and baseball markets. Non-sports activity was limited, with a U.S. Senate midterm market and some weather markets appearing outside the main sports cluster.
What to watch as we round out the month
Kalshi needs roughly $2.82 billion in volume during the week of May 25 to surpass its $14.81 billion April record. That threshold is well below each of Kalshi’s first three weekly totals in May, giving the exchange a clear path to a new monthly high if activity holds near recent levels.
The sports calendar gives Kalshi several near-term catalysts, though some could compress rather than extend trading activity. The Knicks entered Monday night’s Game 4 against Cleveland with a 3-0 lead in the Eastern Conference finals, meaning a New York win would end that series and remove a major NBA market from the weekly board. The Western Conference finals remain active, while the NBA Finals are scheduled to begin June 3.
The NHL conference finals, the French Open, MLB, WNBA and cricket markets give sports traders additional outlets this week, while Texas primary runoffs, midterm and 2028 election markets, the June 16-17 Fed meeting and the June 11 start of the 2026 FIFA World Cup could support activity across sports, politics, economics and futures markets.
