The NFL’s Supreme Court brief said $1.8 billion of $3.3 billion in prediction-market trading on the 2026 season’s opening Sunday was tied to the league. That figure reflects notional trading volume, which measures potential contract payouts rather than the cash traders paid.

What the $1.8 billion measures

USA TODAY tested the claim using trading data and contract details from Kalshi and Polymarket US. Its analysis found that NFL-related contracts made up 59% of the two exchanges’ combined notional volume on Sept. 13, broadly consistent with the NFL’s estimate.

Kalshi and Polymarket together represented about 88% of trading across nine exchanges tracked by DeFi Rate during the 30 days ending Oct. 14. Notional volume counts the total potential payouts associated with contracts traded; it is not a tally of the money participants put down. The distinction matters when comparing prediction-market figures with cash wagered in conventional sports betting.…