To shed some light on how big — and how fast — prediction markets are growing, clock this: Kalshi added almost $20 billion in sports and combo notional trading volume in the last 30 days compared to the prior period, and yet it lost market share.
According to Aldrin Research data, Kalshi’s combined volume in sports and combos rose from $30.2 billion from Aug. 1-30 to $50 billion from Aug. 31-Sept. 29, an increase of 65%. But its share of the combined sports and combo market fell from 76.1% to 73.4%.
Obviously, Kalshi remains the 800-pound gorilla in the room, but Polymarket US, DraftKings’ entry (DKeX), and Novig grew faster, gaining share in both sports and combos.
Across all exchanges listed, combined notional volume climbed approximately 72%, from $39.74 billion to $68.18 billion. This is clearly a result of NCAA and NFL football kicking off, and the vast majority of the growth in volume came from combos. Volume there more than doubled, rising from $18.3 billion to $41.7 billion. Sports volume increased 23%, to $26.5 billion.
All told, combos accounted for 82% of the additional volume between the periods.
For Kalshi, the sports numbers showed steady growth alongside a (mildly) shrinking share. Volume increased 14%, from $13.1 billion to $15 billion, but its share dropped from 61.3% to 56.8%.
The change in market share was even more pronounced in combos. Kalshi’s volume more than doubled to $35 billion, but its share fell from 93.5% to 83.9%. Taken together, Kalshi’s competitors increased their combo volume from $1.2 billion to $6.7 billion.
Polymarket US had the biggest slice of that growth, seeing its combo volume go from $357.1 million to $3.77 billion, boosting its share from a shade under 2% to 9%. Including sports, its volume increased 127%, to $8.3 billion, and its combined share rose from 9.2% to 12.2%.
DKeX posted even larger percentage gains, starting from a much smaller base. Combined volume rose from $88.7 million to $1.9 billion, and its overall share increased from 0.2% to 2.7%. In combo volume, it rose to third place, behind Kalshi and Polymarket.
Novig — perhaps receiving a little Sydney Sweeney boost — also gained ground in both categories. Sports volume more than doubled to $810.3 million, and combo volume nearly tripled to $613.1 million. Combined, Novig grew 136%, lifting its share from 1.5% to 2% of the total market.
Up, down, all around
Other exchanges had more mixed results.
Underdog’s sports volume fell 39%, but its combo volume rose a whopping 263%. That was enough to more than double its combined volume and increase its overall share. Combos accounted for 85% of its September total, compared with 48% in August.
Crypto.com gained sports share, but its slower combo growth left its combined share essentially unchanged. ProphetX also gained sports share while losing a little ground overall.
Polymarket’s results also look different when its U.S.-based and international exchanges are considered together. International volume declined in both categories, partially offsetting the U.S. exchange’s gains. Across both exchanges, combined sports and combo volume grew approximately 68%, but their collective share slipped from 16.8% to 16.5%.
