For example: On an American Roulette wheel there are 36 numbers plus the zero and double zero so there are 38 numbers where the ball can land. When you place a bet on a single number the odds of the ball landing on your number are 37 to 1. If you win the casino pays you 35 for your dollar bet. You keep your original dollar, and are paid an additional $35 returning $36. The difference is two dollars (38 minus 36). Divide the $2 by 38 which is the true odds and you come up with the house edge of 5.26%. So you could actually cover all the numbers on the layout and still lose money.