Position sizing determines how much each trade contributes to your profit or loss. A sound entry can produce a modest gain or an account damaging loss, depending on the exposure behind it. Choosing the direction is only part of the decision. Choosing the amount determines how expensive being wrong becomes.
The practical aim is not to find the largest position your account can support. It is to make each trade’s potential loss consistent with your trading rules, while leaving enough capital to continue after a difficult run. The examples below illustrate the arithmetic; they are not recommended risk levels.
Position Size Is Not the Same as Money at Risk
Position size is the number of shares, contracts or currency units you trade. Planned risk is the amount you expect to lose if your exit works as intended. Those figures are connected, but they are not interchangeable.
For a trade managed with a stop, sizing starts with the distance between entry and the planned exit, plus the cash loss that distance represents per unit. This relationship forms the basis of CME Group’s position sizing framework.
Suppose a hypothetical £10,000 account buys 100 shares at £50, with a planned exit at £49. The position value is £5,000, but the planned price loss is £100 before costs. Buying 300 shares would triple that planned loss to £300. It would not make the trade more likely to succeed.
If the exit instead sits at £48, holding 100 shares puts £200 at risk before costs. Keeping the same quantity does not keep risk constant when the distance to your exit changes.
The Same Trading Results Can Produce Different Account Results
Consider ten hypothetical trades: four winners, each earning twice the amount initially risked, and six losers, each losing that amount. Traders often express these outcomes as +2R and −1R, where R means the initial planned risk.
With a constant £100 risk on every trade, the winners produce £800 and the losers cost £600. The result is a £200 profit before transaction costs.
Now keep exactly the same entries and exits, but risk £50 on each winner and £200 on each loser. Profits total £400; losses total £1,200. The account loses £800. The win rate and trade outcomes have not changed. The amounts assigned to them have.
This example shows why reviewing trade selection alone is insufficient. Compare results in both money and R to separate the quality of the trades from the effect of sizing. The broader relationship between average gains, losses and survival is covered in trading expectancy and risk of ruin.
Larger Positions Make Recovery Harder
A fixed percentage sizing rule reduces the cash amount risked as an account shrinks. However, the percentage chosen still determines how much damage a losing sequence causes.
The following calculation starts with £10,000 and assumes ten consecutive losses. Each loss equals the stated percentage of the account immediately before that trade. Costs and execution differences are excluded.
| Risk per trade | Ending balance | Account decline |
|---|---|---|
| 1% | £9,043.82 | 9.56% |
| 3% | £7,374.24 | 26.26% |
| 5% | £5,987.37 | 40.13% |
Recovery is not symmetrical. A 20% loss requires a 25% gain on the remaining balance to return to the starting point. A 40% loss requires about 66.7%. Increasing size to recover faster also increases the damage from another loss.
Keeping cash risk fixed creates a different problem. Risking £100 represents 1% of a £10,000 account, but 2% after that account falls to £5,000. A rule can look unchanged while becoming more aggressive.
Planned Risk Is Not a Guaranteed Loss Limit
A position sizing calculation depends on its assumptions. For stocks, a stop order becomes a market order once triggered, and its execution price can differ from the stop price. A stop limit order controls the acceptable execution price but may remain unfilled. These distinctions are set out in the SEC investor bulletin on stop orders.
Return to the 100 shares bought at £50. If the intended £49 exit instead fills at £47.50, the price loss becomes £250 rather than £100. Commissions and other applicable charges increase it further.
Stress test the position against worse exits, not just the preferred one. Also review open positions together: three trades with £100 of planned risk each create £300 of combined planned exposure. Different instrument names do not make that arithmetic disappear.
Use Sizing Rules You Can Evaluate
Write down how size changes before placing trades. Otherwise, an increase after a win can be mistaken for confidence, and an increase after a loss can be dressed up as opportunity. Neither is a measurable rule.
A useful review compares alternative sizing methods against the same recorded trades. Test fixed cash risk, a fixed percentage of current equity, and any proposed reductions during drawdowns. Keep entries, exits and cost assumptions unchanged so the comparison isolates sizing.
The practical steps for converting a loss allowance into an order quantity belong in your position sizing and trading risk budget. Your review should then ask whether the chosen rule produces losses you can absorb without changing the strategy midstream.
Record planned risk, actual profit or loss, account equity and departures from the sizing rule. If a position prompts you to abandon a planned exit, review both the rule and whether the exposure was manageable.
Position sizing cannot turn losing trade economics into a dependable advantage. Its job is to control how strongly each outcome affects the account. Judge it by the losses it permits and the consistency it supports, not simply by the biggest profit it could produce.