Most retail investors decide how much to put into a position based on how confident they feel. "This one looks strong, I'll put in S$20,000." Or they use round numbers for simplicity: S$10,000 per trade. Neither approach has anything to do with risk management.
Professional systematic investors size positions based on one question: how much of my total portfolio am I willing to lose if this trade hits my stop-loss? The answer defines the position size. The math is straightforward, repeatable, and removes gut feel from the equation entirely.
The Position Sizing Formula
The formula is:
Position Size = (Portfolio Value × Risk %) ÷ (Entry Price − Stop-Loss Price)
Breaking this down with a Singapore example:
- Portfolio value: S$200,000
- Risk per trade: 1% of portfolio = S$2,000 maximum loss
- Entry price: S$5.00 per share
- Stop-loss price: S$4.60 per share
- Risk per share: S$5.00 − S$4.60 = S$0.40
Position Size = S$2,000 ÷ S$0.40 = 5,000 shares
Dollar value of position = 5,000 × S$5.00 = S$25,000
This is 12.5% of the portfolio in a single position. That is significant concentration, but the risk is mathematically capped at S$2,000 (1% of capital) if the stop is hit. The position size follows from the risk limit, not from conviction or a round number.
What Happens Without This Framework
Compare the same investor using gut-feel sizing. They feel strongly about the trade and put in S$50,000, which is 25% of the portfolio. They set no stop-loss because they are confident.
The stock drops 20% to S$4.00. The loss is S$10,000 — 5% of the entire portfolio wiped on a single position. No stop was in place because position sizing by gut feel and stop-loss discipline are rarely found together. The investor holds, hoping for recovery. It drops further.
The problem was never the trade selection. It was the sizing. The same trade with proper position sizing would have been closed at S$4.60 with a S$2,000 loss — painful but manageable. The gut-feel version became an open wound.
Adjusting Risk for Position Conviction
Not all positions carry the same conviction. The 1% rule applies at the conservative end. Systematic investors sometimes use a tiered approach:
- Tier 1 — High conviction (macro-aligned, sector confirmed, fundamentals strong): 1.5% to 2% risk per trade
- Tier 2 — Moderate conviction (2 of 3 criteria met): 0.75% to 1% risk per trade
- Tier 3 — Speculative or early-stage confirmation: 0.25% to 0.5% risk per trade
The maximum total portfolio risk at any one time should not exceed 10-15%. With 10 open positions each risking 1%, the portfolio faces a maximum simultaneous loss of 10% if every single stop is hit on the same day. In practice, losses are rarely correlated this way, but the calculation sets the ceiling.
Why Stop-Loss Placement Drives the Position Size
The stop-loss is not placed after the position size is determined. It is placed first, based on the chart structure or a technical support level that invalidates the trade thesis if breached. The stop drives the size.
A wide stop (S$1.20 below entry on a S$10.00 stock) produces a smaller position size than a tight stop (S$0.40 below entry). This is correct. A wider stop means more uncertainty in the near-term price movement. The position should be smaller to reflect that uncertainty, not larger because of greater conviction.
Applying This to a Full Portfolio
A S$200,000 portfolio with 10 positions, each sized at 1% risk, has these characteristics:
- Maximum loss if all 10 stops hit: S$20,000 (10% of portfolio)
- Individual position dollar values will vary significantly based on stop distances
- Tighter stops in trending markets produce larger position sizes and greater capital deployment
- Wider stops in volatile markets produce smaller positions and greater cash reserves
The system naturally reduces position sizes during volatile, uncertain markets and increases them during cleaner, trending environments. This is the opposite of what emotional investors do: they buy more during euphoria (wide stops, small positions incorrectly) and sell during fear (when they should be sizing up on recovery signals).
Position sizing is the most underappreciated edge in systematic investing. The entry thesis gets all the attention. The size is what determines whether the portfolio survives long enough to be right.
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Start a Conversation* All figures, percentages, and projections referenced in this article are for illustrative purposes only and are based on historical performance. Past performance is not indicative of future performance. Actual results will vary depending on individual circumstances, market conditions, and the specific products or strategies selected. This article does not constitute an offer, solicitation, or recommendation to buy or sell any financial product. Please consult a qualified adviser before making any financial decisions.