Win/Loss Streak Probability Calculator

Win and loss streaks are an inevitable feature of any trading system, no matter how profitable it is over the long run. Understanding the statistical probability of streaks helps traders size their positions appropriately and avoid abandoning good strategies during normal periods of drawdown.

Probability of a specific streak of length N:

P(exactly N in a row) = Rate^N

Where Rate = win rate for win streaks, or (1 − win rate) for loss streaks.

Probability of experiencing at least one streak of N within S trades:

P(at least one streak in S trades) ≈ 1 − (1 − Rate^N)^(S − N + 1)

Loss streak probability table (50% win rate system):

Win streak probability table (55% win rate system):

Read the middle column of that first table again. A 50% system throws seven losses in a row about half the time within 100 trades. Not as a disaster scenario, as the ordinary behaviour of a coin. Most traders who abandon a system during a drawdown are abandoning it at exactly the point the mathematics said to expect.

The expected worst streak is the number to plan around, and it has a tidy form: over S trades at a loss rate q, the longest run you should expect is roughly log(S) ÷ log(1/q). At a 50% win rate over 500 trades that comes to about 9 consecutive losses. At a 40% win rate it is about 12. Size the position so that streak is survivable and the rest takes care of itself.

Why this matters for position sizing: If you risk 2% per trade and experience a 10-loss streak:

• Drawdown = 1 − 0.98^10 = 18.3% account drawdown: uncomfortable but survivable
• If you risk 5% per trade and experience a 10-loss streak:
• Drawdown = 1 − 0.95^10 = 40.1% account drawdown: dangerous, psychologically devastating

The psychological trap: After a long losing streak, many traders reduce position size or stop trading entirely, exactly when they should continue with normal sizing, assuming the system is sound. After a long winning streak, many traders increase size, just before the inevitable regression to the mean.

How to use this calculator: Enter your historical win rate and the streak length you want to analyze. The calculator shows how likely that streak is per occurrence, and how certain it is to occur over your typical trading volume. If the numbers surprise you, adjust your position size before the streak happens, not during it.

Streak vs. drawdown planning rule: Size your trades so the expected maximum streak (at 90th percentile probability) creates a drawdown you can live with, both financially and emotionally.