Forex Trading Probability: Losing Streaks and Expectancy
Reviewed: . Examples are educational; they are not live trading signals.
A strategy can win less than half its trades and still have positive average returns, or win most trades and lose money overall. To judge it, you need the size of the gains and losses, the costs, and the uncertainty around the estimates. Win rate is one input, not a verdict.
Probability also explains why several losses can appear without a strategy suddenly changing. It does not tell you to ignore losses or increase size after them. The first step is to specify the question and the assumptions behind the calculation.
Start with an explicit model
Suppose each trade has a 50% chance of winning and the outcomes are independent. In that simplified model, one loss does not make the next trade more likely to win. The NIST binomial-distribution reference describes fixed-probability binary trials; real trading may depart from those assumptions when market conditions or execution change.
A larger sample can make an estimate more informative, but it cannot repair biased data, hidden rule changes or a model fitted to the answers. Nor does it guarantee that tomorrow's win probability matches the historical one. Keep those limits visible when using a trading statistic.
Four losses next is different from four losses somewhere
If the loss probability is q, the chance that the next four specified trades all lose is q4. At q = 0.5, that is 0.0625, or 6.25%. The question changes when you ask whether a run of at least four losses appears anywhere during a longer trading period.
| Question under independent 50/50 outcomes | Calculated probability |
|---|---|
| The next four trades all lose | 6.25% |
| At least one run of four or more losses within 20 trades | 47.80% |
| At least one run of four or more losses within 100 trades | 97.27% |
The longer-horizon figures are calculated by tracking the probability of having no completed four-loss run while ending with zero, one, two or three consecutive losses. A win resets that count; a loss advances it, and a fourth loss leaves the no-run group. After the final trade, subtract the remaining no-run probability from one.
This calculation accounts for overlapping opportunities for a streak. Simply multiplying 6.25% by the number of possible starting points can double-count events and even produce an impossible answer above 100%. The table concerns losing runs only, not runs of either wins or losses.
Calculate expectancy after costs
With p as the win probability, W as average gross win, L as average gross loss magnitude and C as average round-trip cost, the expected net result is p × W − (1 − p) × L − C. Use the same unit throughout. If wins and losses are already net of all costs, do not subtract those costs again.
For a hypothetical example, suppose the win rate is 40%, the average gross win is 1.8R, the average gross loss is 1R and average costs are 0.10R. The calculation is 0.40 × 1.8 − 0.60 × 1 − 0.10 = +0.02R per trade. Increasing the cost assumption to 0.15R changes the result to −0.03R.
That small positive first result is an assumption-driven average, not a promise of profit or evidence of a tested edge. Actual outcomes vary, and the estimates of p, W and L have sampling uncertainty. The CFTC notes the importance of forex fees and financing costs; include them when evaluating a record.
Translate a losing run into account risk
If each of five consecutive losses is exactly 1% of the equity immediately before that trade, equity becomes 0.995 of its starting amount. The decline is about 4.90%, assuming no additional costs or slippage. A different sizing method produces a different account path.
This is why a streak probability alone is not a risk-of-ruin estimate. You must also define the failure threshold, horizon, position-sizing method and possible loss sizes. Real losses can exceed their planned size, so use the risk and stop-loss guide alongside the arithmetic.
Use the numbers to ask better questions
Keep gross and net results separate, record rule violations, and compare periods with different conditions rather than assuming all observations are interchangeable. A losing run should trigger the review you planned, not the belief that a win is now owed to you. Our keep, pause or change framework turns that review into a practical decision.
Sources and further reading
Follow the related learning path in Trading Psychology and Testing.
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