John Candy in "Once Upon a Crime" (1992) once said that there is no winning strategy at roulette beside avoiding casino. We are not so categorical and consider options how to win roulette online.
Statistically, the casino wins more often. These are the rules of the game, and the gambling house has a slight house adge over the players and it's absolutely honest and completely legal. This can be easily proved and tested in practice. At roulette, the maximum win is 35:1 with a bet on a single number (Straight Bet). There are 37 numbers (36 + zero) on the European roulette wheel. We place $1 on each, that is a 37 dollars bet and win only $35 and return your $1. Thus, 1 dollar is the casino's house edge.
Anyway, it's easier to play roulette, more fun and it is more profitable than a lottery, for example. Some lotteries, as it is known, were used to pay out only 50% for winnings and still were popular. At roulette, the player gains huge pleasure from the game process and chooses the optimal bet size and its format.
On the Role of the Probability Theory
All random phenomena are described by the theory of probability, and this is the law. Winning numbers in roulette are random (if no one interferes in the process). Thus, knowing the sequence of winning numbers, it is possible to assume with a certain probability which number will be the winning one in the near future.
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Even Money Betting System
Red/Black, Odd/Even, High/Low — these are even money bets.
At the very beginning of the game, the probability that the ball will stop on a red or black section is 18/37, 0.486. The probability that the zero wins is 1/37, 0.027.
If Black wins, then the Red winning odds are increased like 19/37 = 0.736. In the event, that Black wins twice in a row, the red chances are increased even more 19/37 * 19/37 = 0.865. There is no 100% guarantee of winning and never will be, but the chances are still very good.
For better understanding, we present a table with calculations:
Win (in a row)  Odds 

0  0.486486 
1  0.736304 
2  0.864588 
3  0.930464 
4  0.964293 
5  0.981664 
6  0.990584 
7  0.995165 
8  0.997517 
9  0.998725 
10  0.999345 
The more Black wins in a row, the higher the chances are, the next time the ball will stop in a red section. Nevertheless, the probability that the Red will win, will never reach 100%, since there are no guaranteed winnings at roulette.
Red and Black Betting System Statistics
The question arises, how many times in a row can one or another combination win (red or black, even or odd)? Theoretically, the amount is unlimited, however, according to statistics, no more than 10 repetitions. There are only a few cases where repetitions were more than 10 times in a row (of course, we are talking only about honest casinos).
According to the strategy, the player must bet on those combinations for which the probability of winning is much higher. To do this, the player must have access to the winning numbers sequence statistics. In a landbased casino, players do not have access to statistics because the administration will not allow anything to be calculated or recorded. But in online casinos, it's quite possible.
The Martingale Red and Black Betting System
Some believe that the Martingale betting system can guarantee a win at roulette. But this is not so. Any roulette betting strategy can only cut costs and allow you to play longer, that is, increase the chances of winning if you are lucky.
The essence of Martingale's betting strategy is to double the bets in case of a loss. Let's say our first bet is 1 dollar. If we win, we get $1 profit and place $1 as a bet again. In case of a loss, we double the initial one and make a bet of $2 (on winning we return to the initial bet). In the case of the second loss in a row, we make a bet of $4 (with a win it brings a profit of $1).
You can see the further details in the table:
Bet  Total Loss  Winnings  Profit 

1  1  2  1 
2  3  4  1 
4  7  8  1 
8  15  16  1 
16  31  32  1 
32  63  64  1 
64  127  128  1 
128  255  256  1 
256  511  512  1 
512  1023  1024  1 
1024  2047  2048  1 
As we see, in order to win $1 for 10 steps, we must have the bankroll of at least $2,047, which is quite a large sum. In addition, many casinos limit the maximum bet, as well as the ratio of the minimum bet to the maximum one (for example, 1:100). If you started betting $1, then the maximum bet for you is $100. In our table it is the 7 line, as a result, the winning chance is 0.990584. And, according to statistics, the same color can win up to 910 rounds in a row.
To increase the chance of winning and not lose all the money until the round 7, some players use the stretched Martingale betting strategy for red and black. It looks like this:
Bet  Total Loss  Winnings  Profit 

1  1  2  1 
1  2  2  0 
3  5  6  1 
5  10  10  0 
11  21  22  1 
21  42  42  0 
43  85  86  1 
85  170  170  0 
171  341  342  1 
341  682  682  0 
683  1365  1366  1 
How much can you win in online roulette using this betting strategy? As practice shows, you can increase the initial bet by 1020% per hour of the game. However, due to casino restrictions, losses are unavoidable, and such a rate of the revenue growth does not contribute to a rapid enrichment.
Do not forget that the Martingale betting system has a lot of disadvantages, in particular, a small profit on winning, but very large stakes and casino restrictions that will not allow you to double your bet more than 7 times in a row.
Criticism
Opponents of this betting system use the argument that the ball has no memory. That is, they assure, the bet on red can win even 1,000 times in a row, there is no regularity because the probability of winning this or another number is reset after each throw. In other words, the probability that the bet on red is winning is 18/37 = 0.486 and it does not change ever.
Despite such statements by critics, practice shows that probability theory works when playing roulette. However, the limitations of the casino do not provide an opportunity to turn the game into a source of income.
Practical Guidance
Before betting, skip a few rounds to get a certain statistic of the winning numbers succession. Thus, you will be able to push the limit of 7 bet doubles in a row for a couple of steps and increase the chances of winning. The temptation to put all the money on black, if the red wins 9 times in a row, does not lead to anything good.
This betting system makes it possible to earn money at roulette but small amounts which, as a rule, do not pay back the time spent and do not allow to view roulette as a permanent source of income. Moreover, the risk of a big loss does not disappear.
If the casino canceled its max bet limits and you had more money than the casino, then this betting system would guarantee you a stable win. But, having such money, there is no reason to play in the casino just for the sake of winning, is there?