📌 Key Takeaway
The Kelly Criterion shows that the mathematically optimal risk % (e.g. 25% for a 50% win rate / RR 2.0 strategy) is far too large to use directly — apply only 1/10 to 1/25 of full Kelly in practice. The 1% rule is an ultra-conservative Kelly that prioritizes survival over maximum growth, and that trade-off is entirely rational.
“What’s the optimal risk % per trade?” The Kelly Criterion gives this question a mathematical answer. From your win rate and risk-reward ratio, you can compute the “optimal fraction to bet” that theoretically maximizes long-term capital growth.
This article covers the Kelly formula, worked examples, why full Kelly is dangerous, and how the practical “half Kelly” relates to the 1% rule. Reading Risk-Reward Ratio and Risk-Percent Position Sizing first will deepen your understanding.
What Is the Kelly Criterion?
The Kelly Criterion, proposed by physicist John Kelly in 1956, is a formula for the bet fraction that maximizes long-term capital growth rate. It’s widely applied to position sizing in gambling and investing.
f* = (b × p − q) ÷ b f* = fraction of capital to bet (optimal risk %) p = win rate q = loss rate (= 1 − p) b = payoff ratio when you win (risk-reward ratio)
This can also be written f* = p − (q ÷ b). It means “the higher the win rate and the larger the RR, the bigger the optimal risk %.”
Example: 50% win rate, RR 2.0
p = 0.5, q = 0.5, b = 2.0 f* = (2.0 × 0.5 − 0.5) ÷ 2.0 = (1.0 − 0.5) ÷ 2.0 = 0.5 ÷ 2.0 = 0.25 → 25%
So in theory, a 50%-win-rate / RR-2.0 method grows fastest by risking 25% of the account. That’s surely bigger than you expected — and that very magnitude is why you must not use Kelly as-is.
Full Kelly by Win Rate and RR
| Win rate | RR 1.0 | RR 1.5 | RR 2.0 | RR 3.0 |
|---|---|---|---|---|
| 40% | −20% (don’t bet) | 0% | 10% | 20% |
| 50% | 0% | 16.7% | 25% | 33.3% |
| 60% | 20% | 33.3% | 40% | 46.7% |
| 70% | 40% | 50% | 55% | 60% |
A negative f* (e.g. 40% win rate, RR 1.0) means the edge is negative and you shouldn’t bet at all. This is the same “break-even win rate” idea from the Risk-Reward Ratio article.
Why Full Kelly Is Dangerous
The f* (full Kelly) maximizes growth rate, but using it as-is in live trading is extremely dangerous, for three reasons.
Reason 1: Extreme drawdowns
Full Kelly optimizes only growth rate and ignores intermediate drawdown entirely. At 25% risk, losing 50-70% of the account over a few consecutive losses isn’t unusual. Even if it grows fastest in theory, real human psychology can’t withstand that volatility.
Reason 2: Estimation error easily causes overbetting
Kelly assumes you know win rate and RR exactly. But real values are past measurements that may not hold in the future. If you estimate a 55% win rate but it’s actually 48%, full Kelly becomes a severe overbet and Risk of Ruin spikes. The slightest optimism in your estimate makes full Kelly fatal.
Reason 3: The overbetting penalty is asymmetric
Betting below Kelly (underbetting) only slows growth slightly, but betting above Kelly (overbetting) degrades growth rate sharply, and betting too much can even make expected growth negative. So “betting less” is far safer.
The Practical Answer: Half Kelly and Fractional Kelly
For these reasons, pros use “fractional Kelly” — only a portion of full Kelly. The classic choice is half Kelly (1/2 of full Kelly).
Half Kelly has an excellent property: in theory, it retains about 75% of full Kelly’s expected growth rate while cutting volatility (drawdown) roughly in half. Sacrificing a little growth to greatly reduce risk is an extremely favorable trade-off.
| Method | For 50% win / RR 2.0 | Expected growth | Drawdown |
|---|---|---|---|
| Full Kelly | 25% | Maximum (100%) | Extremely large |
| Half Kelly | 12.5% | ~75% | ~half |
| 1/5 Kelly | 5% | ~36% | Quite small |
| 1% rule | 1% | Low but stable | Very small |
The 1% Rule Is an Ultra-Conservative Kelly
In the example above, against full Kelly of 25%, the 1% rule is 1/25 of full Kelly (about 0.04 Kelly) — extremely conservative. Why go so small?
- Win rate / RR estimates are uncertain: a retail trader’s measured win rate has a small sample and wide confidence interval, so you must heavily discount for error
- Win rate varies with regime: as market conditions change, win rate and RR change. They aren’t fixed constants
- Survival comes first: if you prioritize “not going broke” over growth rate, going far below Kelly is rational
So the 1% rule can be understood as “Kelly, made extremely conservative to account for real-world uncertainty.” Kelly indicates an upper-bound reference; in practice you cap risk at a fraction of it. Compute your method’s full Kelly, then keep your actual risk at roughly 1/10 to 1/25 of it — that’s a realistic way to set risk %.
Applying Your Risk % to Actual Lots
Once Kelly helps you settle on “a risk % suited to your method (a fraction of full Kelly),” all that’s left is applying it to every lot. Use the risk-percent position sizing formula to back-calculate the lot from your stop width.
With TraderIsMe’s Auto-Lots Calculation EA, just set your chosen risk % (e.g. 1%, 2%) and it auto-computes the right lot from the stop-loss line. You mechanically enforce “your optimal risk %” derived via Kelly on every trade.
For setup, see Free EAs — Common Setup Guide. For feature details, see Auto-Lots Calculation EA Manual.
Summary
- Kelly Criterion = f* = (b × p − q) ÷ b. Computes the growth-maximizing risk % from win rate and RR
- Full Kelly for 50% win / RR 2.0 is a hefty 25% — dangerous to use as-is
- Why dangerous: ① extreme DD ② estimation error easily overbets ③ asymmetric overbetting penalty
- Half Kelly keeps ~75% of growth while halving DD. Fractional Kelly is the practical default
- The 1% rule is ~1/25 of full Kelly — ultra-conservative. Kelly is an upper bound; cap actual risk at a fraction
- Apply your chosen risk % to lots automatically with Auto-Lots Calculation EA
Kelly teaches us that “there is a mathematical answer to the optimal risk %.” But the wisdom of long-term survivors lies not in using that answer directly, but in discounting it heavily to account for real-world uncertainty.
Related Articles
- The 1% Rule in FX Money Management — The Only Way Pros Stay in the Game — The 1% rule as ultra-conservative Kelly
- Risk-Reward Ratio — Why It Matters More Than Win Rate — The b (RR) in the Kelly formula and expectancy
- Why You Should Drop Fixed Lots — Risk-Percent Position Sizing — Applying your chosen risk % to lots
- Maximum Drawdown Explained — Money Management to Lower Your Risk of Ruin — Why full Kelly’s DD is dangerous
- Auto-Lots Calculation EA — Features and Input Parameters — Auto-applies your optimal risk % to lots