Pick a wheel and spin. The fair-coin mode is the 50/50 most people imagine; the EU and US modes keep the green zeros that make real roulette pay less than it looks.
Why red is never a coin flip
On a European wheel 18 of 37 pockets are red — 48.6%, not 50%. The zero belongs to neither color, and that missing 1.4% is precisely the house edge on every even-money bet. The American second zero widens it to 5.26%.
Streaks are just what independent randomness looks like: after five reds, the sixth spin is still 48.6% red. The gambler's fallacy is betting as if the wheel remembers.
Methodology
Each spin draws a pocket via random_int (CSPRNG): 18 red, 18 black and 1 or 2 green on the real layouts. P(red) is 18/37 (48.6%) European, 18/38 (47.4%) American — the shortfall from 50% is precisely the even-money house edge.
What 2.70% costs in practice
A colour bet on a European wheel returns 97.30% of what you stake across a long run, so the cost of play is the other 2.70%. At £10 a spin and 40 spins an hour that is £400 staked and about £11 expected loss an hour, which is the honest price of the entertainment. On an American wheel the same hour costs £21.
The reason the bet feels close to even is that variance dwarfs the edge over a short session: after 40 spins the likely range of results spans tens of pounds either side, so the edge is invisible. It becomes the only thing that matters as spins accumulate, which is why the house cares about volume and never about a single result.
FAQ
Is red or black a fifty-fifty bet?
Not quite, and the gap is the whole business model. On a European wheel red wins 18 times in 37 — 48.65% — because zero is neither colour. On an American wheel it is 18 in 38, or 47.37%.
Does a long run of black make red more likely?
No. Each spin is independent, so the chance stays 48.65% no matter what preceded it. A run of eight blacks happens roughly once in every 450 spins, and it predicts nothing.
Does doubling after a loss fix the edge?
It cannot. The Martingale wins small amounts often and loses the bankroll rarely, and the two balance out at the same expected loss. Table maximums make the rare loss arrive sooner than the maths suggests.
What is the practical takeaway?
The cost of this bet is 2.70% of everything you stake on a European wheel and 5.26% on an American one. Choosing the wheel matters more than choosing the colour.