Who this is for — Traders who have defined an invalidation point and a risk budget but still need to translate them into shares, contracts, lots or other tradable units.
Trade size or position size is the quantity bought or sold. It is not the same as required margin and does not by itself describe exposure: point value, contract multiplier, currency and leverage can make two positions with the same unit count very different.
In a simplified model, size connects two quantities defined before the order:
- the monetary risk budget allocated to the trade;
- the estimated loss per unit between entry price and invalidation level.
The relationship is:
theoretical units = risk budget ÷ estimated loss per unit
For linear instruments with no multiplier, estimated loss per unit may be approximated by |entry − stop|. Futures, FX instruments and other linear products also require contract specifications, tick value, multiplier and any currency conversion. Options must be treated separately: when invalidation is defined on the underlying, their P&L is nonlinear and also varies with implied volatility, time remaining and other sensitivities. The formula |entry − stop| × multiplier is insufficient; use a consistent repricing scenario or, when applicable, the contract's maximum loss.
From budget to units
Calculation procedure
- Define the point that invalidates the trade under the chosen method; do not move it merely to obtain more units.
- For a linear product, calculate theoretical loss per unit using the applicable distance, tick, multiplier and currency conversion; for an option, use a consistent repricing scenario or the relevant contractual maximum loss.
- Divide the risk-per-trade budget by that unit loss.
- Round to an actually tradable increment without exceeding the planned budget.
- Recalculate risk, commissions, assumed slippage, margin and aggregate exposure before the order; if the total exceeds the budget, reduce size and repeat the check.
Educational example: entry 100, invalidation 97, budget 60 and a unit multiplier produce 20 theoretical units (60 ÷ 3). If the product has a different multiplier, minimum lot or quote currency, the result changes. Even with 20 units, a gap or slippage may take realized loss above 60.
| Quantity | What it describes | What it does not establish |
|---|---|---|
| Size | Number of units or contracts | A guaranteed maximum loss |
| Notional | Nominal value of exposure | Capital actually lost |
| Margin | Collateral required | Total economic risk |
Limit — A stop does not guarantee an execution price. Liquidity, gaps, slippage, commissions, leverage, correlations and currency moves can make actual loss differ from the estimate.
Sources
- CME Group, Proper Position Size — relationship among stop placement, monetary risk and number of contracts.
- CME Group, Position and Risk Management — contract count, tick value, margin and risk scenarios.
- CME Group, Options Premium and the Greeks — shows why an option's price depends dynamically on the underlying, volatility, time and related sensitivities.
- FINRA, stop-order risks in volatile markets — potential difference between stop price and actual execution price.