Streak calculator — exact, both questions

Pick a run length and a session, get both truths instantly: the fixed-window probability (18/37)^N everyone quotes, and the at-least-once-in-your-session probability — an exact Markov-chain computation running live in your browser, the number that actually answers “will I see it?”.⚙ computed

Run of · over spins ·

Reading the two numbers

The fixed-window figure answers “if I point at the next N spins, what’s the chance they’re all red?” — tiny, and the folklore number. The at-least-once figure answers the question you actually live: “across my whole session, does a run this long appear somewhere?” — dramatically larger, because a 200-spin evening contains ~194 overlapping windows. That gap is where the “due” illusion breeds and where Martingale bankrolls die.

Worked example

Seven reds, 200 spins, European wheel: fixed window (18/37)⁷ = 0.635% — yet the session probability is 48.4%: nearly every other evening contains one. Set N=10 and the session figure still reads ~7% — rare, but twenty tables on a floor produce several nightly. Streaks aren’t anomalies; they’re the texture of independence.

Preset pages with commentary and the Martingale tie-in: 3 · 4 · 5 · 6 · 7 · 8 · 9 · 10 · 12 · 15 — or the theory on the odds hub and methodology.

Frequently asked

What run length should surprise me?

Almost none of the plausible ones: even 10 in a row visits ~7% of 200-spin sessions. Genuine statistical surprise starts far beyond anything a session will show you — which is exactly why “the wheel is off” feelings are miscalibrated.

How does this streak calculator work?

A Markov chain tracks the current run length across every spin of your session and sums the probability of ever reaching the target — the exact answer, not a simulation, verified against Monte Carlo to ±0.06%.