On an American wheel, there are 38 spots: numbers 1-36, plus 0 and 00. Your odds of winning a one-number bet are 37 to 1 (37 ways to lose, 1 way to win). But if you win, the casino doesn't pay you 37 to 1, they pay you less: 35 to 1. The difference between the true odds and what they actually pay you is 2/38, or 5.26%. You can do this same calculation for the other bets, and it comes out the same.