European roulette has 36 numbered pockets and one green zero, 37 in all. A straight up bet on one number pays 35 to 1, but the true odds are 36 to 1, and that gap creates the house edge of 1 in 37, or 2.70%. The American wheel adds a double zero, which raises the edge to 5.26%. French roulette uses the same wheel as European and, with its La Partage or En Prison rules on even money bets, can halve the edge on those bets. Always check which wheel is in use.
Example
Betting 1 unit on a single number: the chance of winning is 1 in 37 and the payout is 35 plus the stake back. Over many spins, the expected loss is about 0.027 units per spin.