How position sizing actually works
Most losing accounts are not destroyed by bad analysis. They are destroyed by inconsistent size — a run of small wins followed by one oversized loss that erases them. Position sizing inverts the usual order of thinking. Instead of asking “how many shares can I afford?”, you decide first how much money you are willing to lose if you are wrong, and let that decision dictate the size.
That turns every trade into the same-sized bet regardless of the instrument, its price, or its volatility. A $50 stock with a wide stop and a $500 stock with a tight stop end up risking identical dollars. Your results then reflect your edge rather than the accident of which positions happened to be large.
A worked example
A $25,000 account, risking 1% per trade. Entry at $100.00, stop at $97.50.
- Risk amount: $25,000 × 1% = $250
- Stop distance: $100.00 − $97.50 = $2.50 per share
- Position size: $250 ÷ $2.50 = 100 shares
- Notional exposure: 100 × $100 = $10,000, or 40% of the account
Note the last line. Risking 1% required committing 40% of the account as exposure. That gap between risk and exposure is the single most misunderstood number in retail trading. A tight stop makes a position feel small while the notional is enormous — and it is the notional that suffers on a gap, a halt, or a weekend news event where your stop never gets the chance to work.
Choosing a risk percentage
The arithmetic of drawdown is unforgiving and worth internalising. At 1% risk, ten consecutive losses cost roughly 9.6% of the account. At 5%, the same ten losses cost about 40% — and recovering from a 40% drawdown requires a 67% gain, not a 40% one. Streaks of ten are entirely normal for a strategy that wins half its trades.
This is why professional risk mandates cluster between 0.25% and 2% per position, and why they cap the total risk open across all positions at the same time. Sizing one trade correctly means little if you hold six correlated trades that are effectively one large trade wearing six names.
Tips to consider
The calculation assumes your stop fills at the price you set, which is true most of the time. Knowing the situations where it may not lets you size for them on purpose rather than be surprised by them.
- Gaps in equities. A stock can reopen well away from your stop after earnings or overnight news, and the fill is that opening price rather than the level you chose. Keeping an eye on the notional, not just the risk figure, keeps that outcome comfortable.
- Weekend gaps in forex and indices. Both close and reopen at whatever price the news across the break justifies. Trimming size into a Friday close is the common answer, as is treating the risk number as a good estimate rather than a hard cap when you hold over a weekend.
- Thin or fast-moving markets. When the spread widens quicker than an order can execute, the realised loss runs past the calculated one. A wider stop with smaller size generally travels better here than a tight stop with a large one.
- Scheduled events. Earnings, central bank decisions and major data releases all raise the odds of a gap. Reducing size ahead of a date you already know about costs very little and removes the worst case.
- Let the stop set the size, not the reverse. Place the stop where the trade would be structurally wrong, then size to it. Tightening a stop purely to justify a bigger position is the most common way this calculation gets misused — the size looks disciplined while the odds of being stopped out by ordinary noise climb toward certainty.
