Position sizing without blowing up

October 14, 2026·3 min read

Most investors obsess over what to buy and wing it on how much. That ordering is backwards. Given two investors with the same ideas, the one with the better sizing discipline wins — because sizing is what turns edge into wealth and what turns bad luck into ruin. Here is the statistical framework, stripped of mystique.

Start with expected value

Every position is a bet with a probability distribution. Expected value (EV) is the probability-weighted average outcome: EV = p(win) × gain − p(loss) × loss. If EV is not positive, position size does not matter — the correct size is zero. Most investors never compute this explicitly, which is why they hold positions they cannot defend.

The hard part is not the arithmetic; it is the inputs. Your “60% chance of being right” is a guess, and guesses are systematically overconfident. This is where base rates come in: before trusting your gut, ask how often similar ideas have worked historically. Your stock pick is not special until the base rate says the category works. Shrink your estimated edge toward the base rate, then size.

The Kelly fraction

Given a genuine edge, the Kelly criterion gives the bet size that maximizes long-run compounded growth: f* = (p(b+1) − 1) / b, where p is win probability and b is the net odds (profit per unit staked). A 60% win rate at even money gives f* = 20% of bankroll.

The formula is exact and the inputs are fiction — which is why professionals almost never bet full Kelly. Full Kelly assumes you know p and b. You don't. Estimation error is the norm, and full Kelly punishes overestimation brutally: overbet your edge by 2× and your growth rate goes to zero. The standard practice is half-Kelly or quarter-Kelly: most of the growth, a fraction of the volatility.

Try it yourself with the Kelly position-size calculator — it shows full, half, and quarter Kelly side by side with a drawdown warning.

The drawdown reality

Here is the part Kelly's formula doesn't print in bold: even at the mathematically optimal size, drawdowns are vicious. A full-Kelly bettor should expect to lose half their bankroll at some point with depressing regularity. Half-Kelly cuts the drawdown roughly in half while sacrificing only a quarter of the growth. That trade is the best deal in investing, and almost nobody takes it because it feels too conservative.

Rules of thumb that survive contact with reality: never risk more than you can lose without changing your behavior; size every position as if your edge estimate is half what you think; and remember that ten uncorrelated 2% positions beat one 20% conviction every time — diversification is position sizing's quiet partner.