Position sizing
Family III · Risk
Not to be confused with position size, position trading.
Position sizing is the calculation that converts a risk decision into a specific trade quantity. It answers the question of how large a position may be, given the size of the account, the level at which the trade is proven wrong, and the fraction of capital the trader is willing to lose on that outcome. The result is a number of units, not a direction or a price.
The standard risk-based formula
Most risk-based position sizing uses three inputs: account equity, the maximum acceptable loss expressed as a percentage or fixed amount of that equity, and the per-unit distance between the entry price and the stop-loss price.
The quantity is then:
- Risk amount = account equity × risk percentage
- Per-unit risk = entry price − stop price (for a long trade)
- Position size = risk amount ÷ per-unit risk
Because the stop distance varies with volatility and market structure, the calculated size changes from trade to trade even when the account and risk percentage stay constant.
Worked example
An account holds 50,000 in equity and the trader risks 1% per trade. A long entry is planned at 82.50 with a stop at 79.50, giving a per-unit risk of 3.00.
If the stop were placed at 80.50 instead, the per-unit risk would fall to 2.00 and the same 500 risk budget would support 250 units. Wider stops therefore produce smaller positions, and tighter stops produce larger ones, for the same monetary risk.
Fixed versus volatility-adjusted sizing
Not all sizing methods use a stop distance. Fixed-fractional sizing risks a constant percentage of equity per trade but still requires a stop to define the loss. Fixed-unit sizing trades the same quantity each time, which means the monetary risk varies with the stop distance. Volatility-adjusted sizing scales the quantity inversely with a volatility measure such as average true range, so that each position carries a similar expected fluctuation.
Whatever the method, position sizing is distinct from leverage. A broker may permit a maximum leverage ratio, but that ratio sets a ceiling on quantity, not a recommendation for it. The risk-based calculation usually produces a size well below the leverage limit.
Why the stop must be defined first
Position sizing depends on the stop price, so the stop must be chosen before the quantity is calculated. Choosing a quantity first and then placing the stop at an arbitrary distance inverts the process and makes the loss per trade unpredictable. In markets where a stop cannot be placed at a precise level, such as during a gap or a limit move, the realised loss can exceed the planned risk amount regardless of the calculated size.
Often confused with
- position size
- Position size is the resulting quantity of units, shares, or contracts in a trade, whereas position sizing is the method or calculation that produces that quantity; the visible sign is that position size is a number, while position sizing is a formula or procedure.
- position trading
- Position trading is a style that holds a trade for weeks or months, while position sizing is a risk calculation applied to any holding period; the visible sign is that position trading describes how long a trade is held, not how large it is.