The expected value of a bet in roulette is calculated using the formula: (probability of winning * payout) - (probability of losing * bet size). For example, in American roulette, there are 18 red numbers, 18 black numbers, and 2 green numbers (0 and 00). If you bet $1 on red, the probability of winning is 18/38, or 47.37%. The payout for winning is 1:1, so the expected value of the bet is ($1 * 47.37%) - ($1 * 52.63%) = -$0.053. This means that on average, for every $1 you bet on red, you can expect to lose about 5 cents over the long run. It's important to note that this is just the expected value on average, and that the actual outcome can vary greatly from this average, due to the randomness of the game