Position Sizing: How Much to Bet When You Have an Edge

Most investing advice focuses on what to buy. Very little focuses on how much to buy — and that's a problem, because position sizing is often the difference between a profitable strategy and a catastrophic one.

You can have the best stock picks in the world and still go broke if you bet too aggressively on each one. Conversely, you can have a modest edge and compound it into serious wealth if you size your positions intelligently.

This guide connects the theory from our Kelly Criterion article to practical portfolio decisions. We'll work through real examples with stocks, crypto, and sports betting — and explain why most people systematically overbet.

Imagine two investors. Both identify the same undervalued stock with a genuine 60% chance of returning 50% and a 40% chance of losing 30%.

Investor A puts 80% of their portfolio into it. Investor B puts 15%.

The expected value of the trade is identical for both: (0.6 × 50%) + (0.4 × -30%) = +18%. That's a great bet. But watch what happens over multiple similar bets.

Investor A, betting 80% each time, will eventually hit a losing streak. Three losses in a row takes their portfolio from £10,000 to £10,000 × 0.76³ = £4,390. They've lost 56% of their capital and now need a 127% gain just to get back to even. That's the kind of hole most people never climb out of.

Investor B, betting 15% each time, survives the same losing streak with £10,000 × (1 - 0.15 × 0.30)³ = £8,715. A 13% dip. Uncomfortable, but recoverable.

The edge was the same. The only difference was position sizing.

The Kelly Criterion gives us a mathematically optimal bet size. For a simple binary outcome:

Kelly fraction = (bp - q) / b

Where:

• b = the odds received (net profit per £1 wagered)
• p = probability of winning
• q = probability of losing (1 - p)

Using our stock example: b = 50/30 = 1.667, p = 0.6, q = 0.4

Kelly = (1.667 × 0.6 - 0.4) / 1.667 = (1.0 - 0.4) / 1.667 = 36%

So the Kelly Criterion says to put 36% of your portfolio into this trade. That's the bet size that maximises long-term compound growth.

But here's the catch: full Kelly is extremely volatile. While it maximises the geometric growth rate, the ride is brutal. Drawdowns of 50-80% are normal under full Kelly — mathematically expected, in fact.

This is where fractional Kelly comes in. Most professional bettors and quantitative investors use somewhere between one-quarter and one-half Kelly:

• Half Kelly (f/2): 75% of the growth rate, dramatically less volatility. The standard recommendation.
• Quarter Kelly (f/4): 50% of the growth rate, very smooth equity curve. Good for sleep.

In our example, half Kelly = 18% and quarter Kelly = 9%. Both are profitable. The question is how much volatility you can stomach.

Let's say you have a £50,000 portfolio and you've identified three opportunities:

Stock A — Undervalued retailer

• Your estimated edge: 55% chance of +40%, 45% chance of -20%
• Kelly fraction: (2.0 × 0.55 - 0.45) / 2.0 = 32.5%
• Half Kelly: 16.25% → £8,125

Stock B — Biotech with pending trial results

• Your estimated edge: 35% chance of +200%, 65% chance of -50%
• Kelly fraction: (4.0 × 0.35 - 0.65) / 4.0 = 18.75%
• Half Kelly: 9.375% → £4,688

Stock C — Dividend play with catalyst

• Your estimated edge: 70% chance of +15%, 30% chance of -10%
• Kelly fraction: (1.5 × 0.70 - 0.30) / 1.5 = 50%
• Half Kelly: 25% → £12,500

Total allocated: £25,313 (50.6% of portfolio). The remaining 49.4% stays in cash or index funds — and that's fine. Kelly never says you have to be fully invested.

Crypto markets are particularly instructive because the asymmetry between gains and losses is extreme.

Say you believe Bitcoin has a 40% chance of doubling in the next year and a 60% chance of dropping 40%.

Kelly fraction: (2.5 × 0.4 - 0.6) / 2.5 = 16%

Half Kelly: 8% of your portfolio.

That might feel low if you're bullish on crypto. But consider: this allocation survives Bitcoin going to zero five times over before you're wiped out. Meanwhile, someone who put 50% of their portfolio into Bitcoin at the same odds is facing potential ruin.

The lesson: high-conviction, high-volatility assets demand smaller position sizes, not larger ones. The higher the potential loss, the more conservative your sizing needs to be.

Risk of ruin is the probability that you'll lose enough capital to effectively end your investing career (or at least set it back by years).

For a simple model, the risk of ruin when betting fraction f of your bankroll with probability p of winning is approximately:

Risk of ruin ≈ ((1-p)/p)^(bankroll/unit_bet)

The key insight: risk of ruin drops exponentially as you decrease bet size. Going from 40% of your portfolio per bet to 20% doesn't halve your risk — it might reduce it by a factor of ten or more.

This is why professional poker players, sports bettors, and quant funds are so disciplined about sizing. A 2% edge with 1% position sizes has essentially zero risk of ruin over any reasonable timeframe. The same 2% edge with 25% position sizes has a meaningful probability of blowing up.

Rules of thumb for avoiding ruin:

• Never risk more than 2-5% of your portfolio on a single position (this is well below Kelly for most opportunities, which is fine)
• Use stop losses to cap your maximum loss per trade
• If you're down 20% from your peak, reduce all position sizes by half
• If you're uncertain about your edge (and you probably should be), use quarter Kelly or less

Overbetting is the default human tendency, and there are several psychological reasons:

Overconfidence in edge estimation. If you think your probability is 70% but it's actually 55%, full Kelly on your wrong estimate could be dramatically larger than optimal. Since we're almost always overconfident in our estimates, fractional Kelly provides a crucial safety margin.

Ignoring correlation. Kelly assumes each bet is independent. In reality, your five tech stock picks probably move together. If the market drops, they all drop. You need to account for correlation by reducing total exposure.

Outcome bias. After a winning streak, it's tempting to increase position sizes. But a winning streak doesn't mean your edge has grown — it might mean you've been lucky, and the variance is about to swing the other way.

Social proof. When you see someone on social media bragging about putting their entire portfolio into one stock that doubled, you don't see the thousands of people who tried the same thing and lost 80%. Survivorship bias makes reckless sizing look smart.

Here's a step-by-step approach to sizing any position:

1. Estimate your edge honestly. What's the probability distribution of outcomes? Be conservative — if you think there's a 70% chance of profit, model it as 60%.

2. Calculate Kelly. Use the formula above, or for continuous outcomes, estimate the expected return and variance.

3. Apply a fractional multiplier. Use half Kelly if you're confident in your edge estimate, quarter Kelly if you're not.

4. Cap at 5% of portfolio. Regardless of what Kelly says, no single position should exceed 5% of your total portfolio unless you have extreme conviction and limited downside.

5. Check correlation. If you already have three tech stocks, your fourth tech position should be sized as if the combined tech exposure is one position.

6. Re-evaluate after large moves. If a position doubles, it now represents a larger share of your portfolio. Consider trimming back to your target size.

This framework won't maximise your returns in any single year. But it will keep you in the game long enough for compounding to work — and that's the whole point.

Position sizing is the unglamorous skill that separates professionals from amateurs. It's not exciting, it won't make for good dinner party conversation, and it'll often feel like you're leaving money on the table.

But over a 20 or 30-year investing career, the person who sizes their bets correctly will almost certainly end up wealthier than the person who picks better stocks but bets recklessly.

The maths is clear: the optimal bet is almost always smaller than you think. When in doubt, bet less.

For the theoretical foundation behind all of this, read our guide to the Kelly Criterion. And if you're interested in the cognitive biases that make us systematically overbet, our piece on thinking in probabilities covers the psychological side.