On a fair single-zero roulette wheel, one number has a 1 in 37 chance of winning; red or black has an 18 in 37 chance, about 48.65%. A double-zero wheel adds a losing outcome for these bets. The payout tells you what a winning bet pays, not how likely it is to win. These figures are calculated from the conventional wheel layouts in [published roulette rules](https://www.pacodeandbulletin.gov/secure/pacode/data/058/chapter617a/058_0617a.pdf). This guide is for adults aged 18 and over.
Count every pocket, including zero
A single-zero wheel has 37 pockets: 1–36 and 0. A double-zero wheel has 38: 1–36, 0 and 00. Each has 18 red and 18 black numbers. In the conventional settlement rules used here, zero is a losing result for red, black, odd, even, low and high bets. That is why red is not a fifty-fifty proposition. On a double-zero wheel, its chance is 18 ÷ 38, about 47.37%. One specified number has a 1 in 38 chance, about 2.63%.
Separate winning probability from payout
For these conventional single-zero bets in the [published payout table](https://www.pacodeandbulletin.gov/secure/pacode/data/058/chapter617a/058_0617a.pdf), divide the numbers covered by 37 to calculate the winning probability:
- One number: 1 ÷ 37 ≈ 2.70%; payout 35:1.
- Two adjacent numbers, called a split: 2 ÷ 37 ≈ 5.41%; payout 17:1.
- One dozen: 12 ÷ 37 ≈ 32.43%; payout 2:1.
- Red or black: 18 ÷ 37 ≈ 48.65%; payout 1:1.
Payout ratios above describe the winnings in addition to the returned winning stake. Example: a hypothetical one-unit single-number bet paying 35:1 returns 36 units in total if it wins, including the original unit. A losing bet returns nothing. The stake size is purely for arithmetic, not a suggested amount. The single-number return is also illustrated in an [actuarial paper's roulette example](https://www.actuaries.org.uk/system/files/documents/pdf/pricing-risk-1998.pdf), section 5.3.
Work out the house edge
House edge describes the mathematical advantage built into a game: the average share of stakes the house expects to retain. The [Gambling Commission explains house edge](https://www.gamblingcommission.gov.uk/public-and-players/guide/return-to-player-how-much-gaming-machines-payout). This is an average, not a separate fee or a cap on what you can lose.
These are calculated averages under the stated assumptions. They do not predict a session's result. On one spin, that example bet either wins one unit or loses one unit; it never loses exactly 0.027 units. Nor does 1 in 37 mean a particular number must appear once during the next 37 spins.
A results display does not make a number due
For independent spins, earlier results do not change the next spin's probabilities. A run of one colour therefore supplies no reason to increase a stake. Great Britain's [remote randomness standard](https://www.gamblingcommission.gov.uk/standards/remote-gambling-and-software-technical-standards/rts-7-generation-of-random-outcomes) requires acceptably random results and prohibits compensated behaviour that changes probabilities during play. A history display is a record, not a forecast.
A checklist for reading the rules
For remote games regulated in Great Britain, the [Gambling Commission's information standard](https://www.gamblingcommission.gov.uk/standards/remote-gambling-and-software-technical-standards/rts-3-rules-game-descriptions-and-the-likelihood-of-winning) requires rules, information about winning chances and payout details to be accessible before a customer gambles. Use these questions when reading them:
- How many pockets are there, including every zero? Write down the total.
- Which outcomes count as a win for the particular bet? Count those separately.
- What does the payout ratio mean, and does the display include the returned stake?
- What happens on zero? Check for any special settlement rule before using the calculations above.
- Are there extra features, different payouts or restrictions? Read their separate rules; if something remains unclear, leave the game alone.