Single-zero example
On an ideal single-zero wheel there are 37 equally likely pockets. For a straight-up bet paying 35 to 1 in profit, a winning stake of 1 returns 36 including stake. Expected return is 36 divided by 37, about 97.30%; the house edge is about 2.70%.
Double-zero example
With 38 equally likely pockets and the same straight-up payout, expected return is 36 divided by 38, about 94.74%. That is a different mathematical contract, even if both games are labelled roulette.
Read special rules
Zero treatment on even-money bets, special payouts and side bets can change the relevant calculation. The simple example does not describe every roulette variant or predict the next spin.
A straight-up number example
On a standard single-zero wheel, one winning number out of 37 corresponds to a probability of 1/37. A one-unit straight-up stake paying 35 to 1 returns 36 units on a win when the original stake is included. The other 36 numbers lose the unit.
The expected net outcome in that model is (35 − 36) / 37, or approximately −0.027 units per unit staked. On a 38-pocket wheel with the same payout, it becomes (35 − 37) / 38, approximately −0.0526. These figures describe the stated rules over repeated trials, not a short-session forecast. Check variants with special payouts separately.