The Kelly Criterion: How Much Should You Risk on Each Trade?

With gold down 2.8% to $4,651 and silver plummeting 4.1% to $72.74 today, many traders are wondering: how much should I actually risk on each position? While crude oil surges 11.4% to $111.54, the stark contrast in performance highlights why proper position sizing isn't just important—it's survival.

What Is the Kelly Criterion?

The Kelly Criterion, developed by John Kelly Jr. at Bell Labs in 1956, provides a mathematical formula for optimal position sizing. It tells you exactly what percentage of your capital to risk on each trade based on your strategy's historical win rate and average win/loss ratio.

The formula is deceptively simple:

f = (bp - q) / b

Where:

Real-World Application

Let's examine one of our top-performing strategies from RetailVest's leaderboard: the `spx_golden_cross` strategy, which has delivered 1,644% total returns. If this strategy historically wins 55% of trades with an average win-to-loss ratio of 1.8:1, the Kelly calculation would be:

f = (1.8 × 0.55 - 0.45) / 1.8 = 0.3 or 30%

But here's the kicker—betting 30% per trade would be financial suicide for most retail traders. This is where "fractional Kelly" comes in.

The Fractional Kelly Approach

Smart traders typically use 25-50% of the Kelly recommendation. Why? Because Kelly assumes:

In reality, markets evolve. Today's VIX sitting at 17.68 suggests relatively calm markets, but we've seen how quickly that can change. The 2s10s spread at just 0.4% indicates potential economic uncertainty ahead.

Building Kelly Into Your Strategy

Using RetailVest's Strategy Builder, you can backtest Kelly-based position sizing across different market conditions. Here's how to implement it:

1. Calculate your strategy's win rate over the last 100+ trades

2. Determine average win/loss ratio from your backtesting data

3. Apply the Kelly formula but use only 25-50% of the result

4. Adjust for correlation between positions

The Precious Metals Example

Consider the `gold_silver_ratio` strategy (1,058% total returns). Gold and silver often move together, but today's performance shows silver's higher volatility (-4.1% vs -2.8%). If you're running both gold and silver strategies, your effective position size should account for correlation—you're not actually diversifying risk as much as you think.

On RetailVest's Metals page, you can track these correlations in real-time and adjust your Kelly calculations accordingly.

Common Kelly Mistakes

Over-betting: Using full Kelly or higher during winning streaks

Under-betting: Using tiny positions that can't compound meaningfully

Ignoring correlation: Treating correlated positions as independent

Static sizing: Not adjusting as win rates and ratios evolve

When Kelly Breaks Down

The Kelly Criterion assumes your edge remains constant, but markets evolve. The `gold_200ma_trend` strategy's 664% total return might face headwinds if gold continues consolidating around current levels. RetailVest's Insights feature helps identify when strategy performance might be diverging from historical norms.

Risk Management Beyond Kelly

Even with optimal Kelly sizing, never risk more than you can afford to lose on any single trade. With crude oil's 11.4% surge today, momentum traders might be tempted to increase position sizes—this is exactly when Kelly discipline matters most.

Consider maximum drawdown limits:

Your Action Plan

This week's specific insight: With the S&P 500 up 0.5% to 7,431 while precious metals decline, we're seeing sector rotation that favors trend-following over mean-reversion strategies. If you're running the `silver_rsi_bounce` strategy (645% total return), consider reducing your Kelly percentage by 25% during periods when metals show persistent weakness relative to equities.

Start by calculating Kelly percentages for your three best strategies using RetailVest's backtesting tools, then implement fractional Kelly sizing at 25% of the calculated amount. Your future self will thank you.