Position Sizing Methodology

Risk note: educational content only. This article is not financial advice, investment advice, or a recommendation to buy or sell any instrument. Trading leveraged products can result in losses greater than expected, and each reader should understand the risks before trading.

Position sizing is the calculation that converts a risk management principle into the specific lot size entered on the order ticket. The principle of the previous lesson, that each trade risks a fixed percentage of equity, has no practical effect unless it is translated into a number before every position is opened. The calculation involves three inputs: the monetary risk the trader has decided to accept, the distance to the stop-loss in pips, and the value of one pip for the instrument concerned. This lesson sets out the formula, demonstrates it with a worked example, distinguishes position size from margin requirement, and addresses the concealed risk of correlated positions.

The position-sizing formula

The sequence of decisions is fixed and its order matters. The trader first determines the monetary amount that may be lost on the trade, as a percentage of current account equity. The trader then determines, from the chart, the price at which the trading idea is invalidated and measures the distance from the intended entry to that price in pips. Only then is the position size calculated, as the number of lots for which the loss over that pip distance equals the predetermined monetary risk.

Expressed as a formula: position size in lots = monetary risk ÷ (stop-loss distance in pips × pip value per lot). The size is therefore an output of the calculation and never an input. A wider stop-loss produces a smaller position and an identical monetary risk; it does not produce a larger risk. A trader who begins with a lot size and then selects a stop that accommodates it has reversed the sequence and has no control over the actual risk taken.

A worked example

Consider an account with equity of USD 10,000 and a risk rule of one per cent per trade. The monetary risk is therefore USD 100. The trader identifies a long entry in EUR/USD with a stop-loss 50 pips below the entry. Dividing USD 100 by 50 pips gives a permissible loss of USD 2 per pip. For EUR/USD, one standard lot of 100,000 units has a pip value of approximately USD 10. The position size is therefore USD 2 ÷ USD 10 = 0.20 lots.

If market structure requires the stop-loss to be placed 100 pips from the entry instead, the permissible loss per pip falls to USD 1 and the position size falls to 0.10 lots. The monetary risk on the trade is USD 100 in both cases. The minimum trade size on Onys Kapital accounts is 0.01 lot, so the calculated size is rounded down to the nearest 0.01 lot; rounding down ensures that the actual risk never exceeds the intended risk. The profit calculator on the Onys Kapital website performs this arithmetic for any instrument and lot size.

Margin requirement as a separate calculation

The margin required to open a position is frequently confused with the risk on the position. They are unrelated quantities. Margin is the deposit the broker holds as collateral while the position is open, and is determined by the notional value of the position and the applicable leverage. Risk is the amount that will be lost if the stop-loss is executed, and is determined by the position size and the stop distance.

Onys Kapital applies dynamic leverage based on aggregate position size: 1:1000 for 0 to 5 lots, 1:500 for 5.01 to 10 lots, 1:250 for 10.01 to 20 lots, 1:100 for 20.01 to 50 lots, and 1:50 above 50 lots. The required margin is calculated as (Lots × Contract Size × Market Price) ÷ Leverage. For the 0.20-lot EUR/USD position above, at a market price of 1.1000 and leverage of 1:1000, the required margin is (0.20 × 100,000 × 1.1000) ÷ 1000 = USD 22. The risk on the position remains USD 100 regardless of leverage, which affects only the capital committed as margin.

Correlated positions and concealed exposure

Positions in different instruments may depend on the same underlying factor. Long positions in EUR/USD, GBP/USD and AUD/USD, each sized to risk one per cent, are not three independent one-per-cent risks. Each is a position that profits from a weakening US dollar, and in conditions of dollar strength all three are likely to incur losses simultaneously. The trader has, in effect, taken a single position risking approximately three per cent of equity.

The correlation between instruments is not constant and tends to increase during periods of market stress, which is precisely when it is most damaging. A risk framework should therefore include a cap on aggregate risk across correlated positions, for example a maximum of two per cent of equity in positions that share a common directional exposure. The alternative is to discover the correlation on the day when all the positions lose together.

Procedure and supporting tools

The calculation should be performed before the order ticket is opened and before the chart has been examined for reasons to increase the size. The trader's degree of confidence in a particular setup is not evidence about its outcome and is not a valid input to the calculation; historical records consistently show that the trades about which traders feel most certain do not perform better than the remainder. A position sized upward on conviction has abandoned the fixed-fraction rule.

Onys Kapital provides a profit calculator on its website that computes pip value and profit or loss for a given instrument, lot size and price movement, together with a compounding calculator for projecting the effect of consistent risk over time. The MetaTrader 5 order ticket displays the margin requirement before the order is submitted. A trader is encouraged to practise the full procedure on a demo account until it is performed as a matter of routine.