“How much should I put into this?” matters as much as “should I buy this at all?” — two investors can agree on the exact same idea and have completely different outcomes purely from how much of their money they risked on it.
Before any formula, the practical starting point is simple: no single position should be large enough that being wrong about it would be financially painful. A common, non-personalized heuristic beginners use is to cap any single position at a small, fixed share of the total portfolio (for example, a hard ceiling like 10%) regardless of how confident they feel — confidence in an idea is not the same as the idea being correct, and sizing discipline is what protects you when it isn’t.
Quantustik’s position-size figures come from the Kelly criterion, a formula that turns a historical win rate and a win/loss ratio into a bet fraction. Two details matter for a beginner: first, Quantustik uses half-Kelly — only half of what the full formula would suggest — because the full formula, while mathematically growth-optimal, produces swings most people can’t stomach in practice. Second, a negative edge always floors the size to 0% — the formula itself says “don’t bet” rather than suggesting a small position anyway.
Illustrative example only, not a recommendation: someone with a $1,000 starting portfolio who follows a 10% per-position cap would put at most $100 into any single idea, no matter how promising it looks. If that one idea is wrong, the portfolio absorbs a manageable loss instead of a portfolio-ending one; if ten such ideas all went to zero simultaneously (an extreme, illustrative scenario), the capped investor would still have some capital left to keep investing, while an investor who concentrated the same $1,000 into one or two ideas would not have that cushion.
The previous lesson covered why spreading capital across holdings reduces risk at the portfolio level. Position sizing works at the individual-trade level — how much goes into any one of those holdings. Both disciplines exist for the same reason: to keep any single decision, right or wrong, from doing outsized damage to the whole portfolio.
This lesson is investor education, not personalized advice. The per-position cap discussed is a general heuristic, not a recommendation for your specific situation, and the dollar example is illustrative only.
No. The practical beginner takeaway is simpler than the formula: cap any single position at a small, fixed share of your portfolio regardless of how confident you feel, so being wrong about one idea never does portfolio-ending damage.
Full Kelly is mathematically growth-optimal but produces large, hard-to-tolerate swings in practice. Half-Kelly gives up some long-run theoretical growth for a large reduction in volatility.
It means the sizing formula found no positive historical edge for that specific idea at that time — the math is declining to size a bet at all, which is a different (and safer) outcome than sizing a small bet anyway.
Investor education only — not investment advice, and never a promise of profit. Every investment can lose value.