Trading
Position Sizing: Turn a Risk Budget into Trade Quantity
Calculate position size from a hypothetical risk budget, stop distance, and costs while accounting for execution and portfolio risk.
Position sizing connects a proposed trade with the amount of exposure it creates. It answers how many shares or contracts fit the plan, given the intended exit and a chosen risk budget. It does not ensure that the eventual loss stays within that budget.
That last distinction matters. Price gaps, slippage, liquidity problems, and product leverage can make actual results differ from the calculation. Use sizing as a planning tool built around explicit assumptions, then evaluate what happens if those assumptions fail.
Define the budget before the quantity
A risk budget is the amount allocated to a possible loss under the plan. It is different from the cash needed to buy the position and different from margin required by a broker. CME's position-sizing lesson connects that budget with the location of the planned stop.
A budget can be expressed in dollars or as a fraction of account equity. No single percentage is suitable for everyone or every strategy. Capital needs, drawdown tolerance, existing exposure, and the product's behavior all matter. A small-looking percentage can still create substantial aggregate risk when several positions share the same market driver.
For illustration only, suppose an account has 20,000 in equity and a hypothetical planning budget of 0.5%, or 100. This example explains arithmetic; it is not a recommended account size or risk allocation. Keep the budget in the same currency as the loss calculation, or include a documented currency conversion.
Calculate planned loss per unit
For a straightforward long share position, the entry price minus the planned stop price gives the price distance per share. A hypothetical entry at 50 and a stop at 49.20 create a distance of 0.80. Dividing the 100 budget by 0.80 gives 125 shares before trading costs.
Those shares have a purchase value of 6,250. The purchase value is not the same as the planned 100 price loss. Both numbers matter: the account must support the position, while the risk calculation must describe the consequences of the intended exit.
For a short share position, use the distance from entry to the planned higher stop. For futures, translate the distance into dollars using the correct contract multiplier or tick value. Options require attention to their payoff and sensitivity; do not transfer a stock-price distance directly into an option-loss estimate.
Include costs without false precision
Suppose the same hypothetical trade uses an allowance of 0.10 per share for round-trip costs and slippage. The assumed loss per share becomes 0.90. Dividing 100 by 0.90 and rounding down produces 111 shares, with an estimated loss of 99.90 under those assumptions.
This simplified allowance is not a forecast of actual execution. Some charges are fixed per order, others vary with quantity, and slippage can be much larger during a gap. Where a fee is fixed, subtract it from the available budget before dividing by the variable loss per unit.
Round down to the product's permitted trade increment. If the minimum tradable quantity exceeds the budget, the proposal does not fit as written. Increasing the budget just to make the order possible defeats the purpose of establishing it first.
A stop does not create a guaranteed maximum
Investor.gov describes how a stock stop becomes a market order when triggered and can execute at a different price. In the example, an exit at 48.70 instead of 49.20 adds 0.50 of loss per share. For 111 shares, that is another 55.50 before any additional costs.
A stop-limit order introduces a different tradeoff: it controls the acceptable execution price but may remain unfilled. The SEC's order-types bulletin explains that distinction. Learn the broker's trigger rules, eligible sessions, and order handling before assuming a protective order operates under every condition.
Position sizing should therefore include a stress question: what would a materially worse exit do to this account? That question can change whether a trade belongs in the plan at all, especially when holding through an event or using leverage.
Check the whole account
Three positions each budgeted at 100 do not become independent merely because their symbols differ. They may react together to the same sector news or market move. Add up intended losses and consider shared exposures, available cash, and the effect of simultaneous adverse exits.
Recalculate quantity if the entry or exit changes before the order. Moving a stop farther away while keeping quantity unchanged increases planned risk. Adding to a position requires a new assessment of total exposure, rather than treating the additional order as a separate harmless trade.
A useful sizing record includes equity used, budget, entry, invalidation level, product value per price unit, cost assumptions, rounded quantity, and a stress scenario. The purpose is to make exposure deliberate and reviewable. Even careful sizing cannot make an unsuitable or untested strategy safe.