Quant Trading Academy Module 3, Part 7

You can have a genuine edge and still lose all your money. Position sizing is how.

Most traders spend the majority of their time looking for better entries, better signals, better setups. Almost none spend equivalent time on the question of how much to risk per trade. This is backwards. The quality of your signal determines whether your strategy is profitable in theory. Your position sizing determines whether that theory ever shows up in your account.

Quant provides the signal. Position sizing is your responsibility. This post covers the core frameworks, the maths that underlies them, and the practical approach that gives your edge the best chance of compounding over time.

Why Sizing Matters More Than You Think

Consider two traders using identical Quant signals over 100 trades. The strategy has a 60% win rate and a 1:1 risk to reward ratio, producing a theoretical profit over that sample.

Trader A risks 2% of their account per trade. After 100 trades, their account has grown steadily, surviving all losing streaks without material damage to their capacity to keep trading.

Trader B risks 20% per trade. Their first four consecutive losses, which is a statistically normal occurrence in a 60% win rate system, draws down their account by approximately 59%. From that point, Trader B needs a 144% return just to get back to flat. They are not losing because the signal is wrong. They are losing because their sizing gave variance no room to breathe.

The signal is identical. The outcome is entirely different.

The Three Main Sizing Approaches

Fixed Dollar Sizing

The simplest approach: you risk the same dollar amount on every trade regardless of account size. If your account is $10,000 and you risk $200 per trade, that remains $200 whether your account grows to $15,000 or shrinks to $7,000.

The advantage is simplicity. The disadvantage is that $200 represents a different percentage of your capital as your account fluctuates, which means your risk is not proportionally consistent. As your account grows, fixed dollar sizing becomes increasingly conservative. As it shrinks, it becomes proportionally more aggressive at exactly the wrong time.

Fixed Fractional Sizing

You risk a fixed percentage of your current account on every trade. If you risk 1% per trade and your account is $10,000, you risk $100. If your account grows to $12,000, you risk $120. If it falls to $8,000, you risk $80.

This approach scales naturally with your capital. Winning periods compound your gains proportionally. Losing periods reduce your absolute risk in line with your reduced capital, which slows drawdowns in a mathematically meaningful way. This is the most widely used approach among systematic traders and the one most appropriate for the majority of Quant users.

The practical question is what percentage to use. Most professional frameworks settle between 0.5% and 2% per trade. The right number depends on your risk tolerance, the number of simultaneous positions you might hold, and the maximum drawdown you can psychologically and financially sustain.

Kelly Criterion

The Kelly Criterion is a formula that calculates the theoretically optimal fraction of capital to risk on each trade to maximise long run growth. Given a win rate and a risk to reward ratio, it produces a specific percentage.

For a 60% win rate strategy with a 1:1 risk to reward:
Kelly fraction = (0.60 × 1 - 0.40) / 1 = 0.20, or 20% of capital per trade.

In theory, this maximises compounding. In practice, full Kelly is almost universally considered too aggressive for real trading conditions. The formula assumes perfect knowledge of your true edge, which you never have. It assumes no variation in win rate or payoff across trades. And it produces drawdowns that are psychologically unbearable for most people, even when mathematically survivable.

The practitioner's standard is half Kelly or quarter Kelly. At half Kelly in the above example, you would risk 10% per trade. Many professional traders apply a further discount, landing somewhere in the 1% to 3% range, because real world trading conditions always introduce more uncertainty than the formula accounts for.

For most Quant users, fixed fractional sizing between 1% and 2% per trade is the practical sweet spot: conservative enough to survive extended losing streaks, aggressive enough to compound meaningfully over time.

Working Backwards from Risk

The correct sequence for sizing a trade is as follows.

Decide how much of your account you are willing to lose if the trade hits its stop.
Example: 1% of a $10,000 account is $100.
Identify the stop distance for this specific setup.
If the stop is 4% below your entry price, a $100 risk tolerance means your position size should be $2,500.
Determine the leverage required to hold that $2,500 position with your allocated margin.
This number follows from the first two. It is not chosen independently.

This sequence protects you from the most common sizing error: choosing a leverage multiple first and then working out what it implies for risk. That approach produces position sizes that reflect your appetite for leverage, not your actual risk tolerance.

Ruin Risk and Why Conservatism Compounds

Ruin risk is the probability that a sequence of losses reduces your account to a level from which recovery is no longer realistic. It is not a remote theoretical concern. It is the natural endpoint of aggressive sizing applied to a volatile instrument.

The mathematics are clear: the more you risk per trade, the higher your ruin probability, even with a positive expected value strategy. A 2% risk per trade and a 60% win rate produces a near-zero ruin probability over thousands of trades. Scaling that to 10% per trade, the ruin probability increases sharply even though the edge is identical.

Conservative sizing also has a compounding benefit that is easy to overlook. Smaller losses are proportionally easier to recover from. An account that never suffers a catastrophic drawdown compounds more efficiently over time than one that swings violently between extremes, even if the average return per trade is identical.

The goal is not to maximise any single trade. It is to remain in the game long enough for the edge to express itself across hundreds of trades. Position sizing is the mechanism that keeps you there.

Key Takeaways

Position sizing determines whether your edge shows up in your account. Signal quality is necessary but not sufficient.
Fixed fractional sizing, risking a consistent percentage of current capital per trade, is the most appropriate approach for most traders.
Between 1% and 2% risk per trade is the practical range that balances growth with survival through normal variance.
Always work backwards from your risk tolerance to your position size. Never start with a leverage multiple.
Conservative sizing compounds more reliably than aggressive sizing, because large drawdowns are disproportionately difficult to recover from.
Ruin risk is real and increases sharply with position size, even in a provably +EV strategy.