The full guide
Position sizing for CSE traders — the 1–2% rule, stop distances, and the friction nobody prices in
Reviewed and updated July 16, 2026 · Written for Sri Lankan investors and borrowers
Ask a profitable trader their secret and the honest answer is usually boring: they never let one trade hurt them. Position sizing — deciding how many shares to buy before you buy them — is the mechanism. It converts a vague intention to be careful into an exact share count derived from your account size, your risk tolerance, and your stop-loss.
This guide explains the risk-based sizing method, then adapts it to Colombo Stock Exchange realities: a 1.12% transaction cost each way, thin liquidity on smaller counters, and the concentration problem in a market where a handful of stocks dominate turnover.
The 1–2% rule: cap the damage first
The rule is simple: risk no more than 1% to 2% of your total capital on any single trade. Risk here means the amount you lose if your stop-loss is hit — not the size of the position. With a Rs. 1,000,000 portfolio and a 1% rule, your maximum acceptable loss per trade is Rs. 10,000.
The point is survival through losing streaks, which every trader has. Ten consecutive 1% losses leave you with about 90% of your capital and full ability to continue. Ten consecutive 10% losses leave you with about 35% and needing a 186% gain just to recover. Small per-trade risk is what makes a strategy’s long-run edge actually reachable.
From risk budget to share count
Position size equals your risk budget divided by the loss per share if stopped out. Suppose you want to buy at Rs. 50.00 with a stop-loss at Rs. 46.00 — Rs. 4.00 of risk per share. With a Rs. 10,000 budget, you can buy 2,500 shares, a Rs. 125,000 position (12.5% of the portfolio).
Notice what the formula does: a tighter stop allows a larger position for the same rupee risk, and a wider stop forces a smaller one. Your position size flexes with the trade setup while the potential damage stays constant. Never work backwards — deciding you want a big position and then placing the stop artificially tight is how stops get hit by ordinary noise.
CSE friction: the 1.12% each way changes your numbers
On the CSE, equity trades up to Rs. 100 million cost 1.12% in total charges on the buy and again on the sell — brokerage 0.640%, CSE fee 0.084%, CDS fee 0.024%, SEC cess 0.072%, and the 0.300% share transaction levy. A stopped-out trade therefore loses more than the raw price distance.
Rework the example with costs. Your true entry is Rs. 50.00 times 1.0112, about Rs. 50.56 per share. If stopped at Rs. 46.00, you net Rs. 46.00 times 0.9888, about Rs. 45.48. Real loss per share is roughly Rs. 5.07, not Rs. 4.00 — so the Rs. 10,000 budget supports about 1,971 shares, not 2,500. Traders who ignore friction routinely risk 25% more than they think, and the round-trip drag of about 2.27% also means very short-distance trades on the CSE rarely make sense.
Liquidity: the invisible constraint on smaller counters
Outside the most actively traded CSE names, daily volumes can be thin enough that your position size is constrained by the market, not your risk budget. A stop-loss is only as good as your ability to sell at something near the stop price — in an illiquid counter, a stop at Rs. 46 may fill at Rs. 43 or not at all on a bad day.
A practical discipline is to size positions so you could exit within a few days at recent average volumes, and to treat any position larger than a small fraction of typical daily turnover as carrying extra, unmodelled risk. Wide bid-ask spreads on quiet counters are a second hidden cost that stacks on top of the 1.12%.
Concentration caps and portfolio heat
Per-trade risk control is not enough if all your trades are the same bet. Correlated positions — three banks, or three stocks that all rise and fall with import demand — can lose together. Two portfolio-level limits keep the 1–2% rule honest.
Portfolio-level limits worth enforcing
- Single-position cap: no one holding above roughly 10% to 15% of portfolio value, whatever the stop distance allows.
- Portfolio heat: total open risk across all positions (sum of each trade’s stop-loss loss) capped near 5% to 6% of capital.
- Sector cap: limit combined exposure to any one sector, since CSE sectors often move together on macro news.
- Liquidity test: confirm you can realistically exit each position within a few normal trading days.
Sources & further reading
This guide is educational and reflects publicly available rules and market conventions at the review date. Tax rates, bank rates, and regulations change — verify current figures with the institution or the Inland Revenue Department before making a financial decision. Nothing here is financial, tax, or investment advice.