Every professional trader eventually converges on the same conclusion: the size of your bet matters more than the direction. This is counter-intuitive to new traders, who spend most of their energy on entry signals and almost none on sizing. The math explains why they have it backwards.
01 — Why direction is a distraction
Consider a trader with a 55% win rate — above average for any retail book. If she risks 10% of capital per trade, her probability of hitting zero across a career of 1,000 trades approaches certainty. The edge evaporates, not because the system failed, but because the position size was too aggressive relative to the edge.
A trader with a 52% win rate but 1% risk per trade, by contrast, can survive indefinitely and compound steadily. Same edge, radically different outcome — because of sizing alone.
"The first requirement of a good strategy is that it does not kill you while it is being correct."
02 — The Kelly criterion, simplified
The Kelly criterion formalises the relationship. For a trade with win probability p and win/loss ratio b, the optimal fraction f of capital to risk is:
Where q = 1 − p. For most retail systems, Kelly produces an uncomfortably high number — 15-25% of capital per trade is not unusual on paper. Almost every professional trader uses "fractional Kelly" — typically a quarter or an eighth of the full Kelly fraction — to account for model error and the psychological cost of drawdowns.
03 — Position sizing in practice
In MT5, position sizing for a given risk amount collapses to a simple formula:
- Decide the dollar risk per trade (e.g. 1% of account equity)
- Measure the stop distance in pips
- Divide dollar risk by pip value × stop distance
- The result is your position size in lots
A $10,000 account risking 1% on a 50-pip stop in EUR/USD: $100 risk ÷ ($10 pip value × 50 pips) = 0.2 lots. That is the maximum size consistent with the risk budget. Size smaller if conviction is lower — never larger.
04 — When to override the math
There are exactly two conditions under which deviating from the sizing framework is defensible. First, reducing size in periods of elevated uncertainty — the math assumes the edge is stationary, and it almost never is. Second, reducing size after a drawdown to preserve capital for the next regime.
Increasing size beyond the framework is never defensible. The trader who deviates upward is not confident — they are capitulating to the market's current narrative. The framework exists precisely to protect you from that impulse.
05 — Implementing in MT5
MT5 has a built-in lot-size calculator accessible from the order window, but serious traders implement position sizing as an Expert Advisor that reads account equity, a predefined risk percentage, and the distance from entry to stop. The logic is ~20 lines of MQL5. A reference implementation is available in our Toolkit section.
06 — Summary
Professional trading is not a game of prediction. It is a game of survival, compounded over a long enough horizon that the edge can express itself. Position sizing is how you buy the tickets to play. Buy expensively and one bad streak ends the career. Buy cheaply and the edge has time to work.