In roulette, the house edge can be calculated by looking at the probability of winning versus losing on a simple bet like red or black. With 18 red numbers and 18 black numbers, plus one green zero, there are 18 out of 37 chances to win and 19 out of 37 chances to lose when betting on either red or black. The extra green zero gives the casino an additional chance to win, which is 1 in 37, or approximately 2.7%. This percentage represents the house edge in roulette for these types of bets.