Position sizing: how to calculate trade size and risk

Position sizing means deciding how much exposure to take in a trade. It connects the amount you trade with the loss you are planning for if the market moves against you. The amount of margin needed to open a position is a separate calculation.

A useful starting point is a loss budget and an intended exit level. From those, you can calculate a quantity. This is a planning tool: an exit may fill at a worse price, costs may change, and liquidation can occur before an intended exit.

Quantity, notional, and margin are different

Quantity is the number of asset units or contracts. Notional is the value of the exposure. For a simple linear position measured in asset units, entry notional equals quantity multiplied by entry price.

Margin is collateral supporting the position. Reducing the margin allocated to the same exposure does not reduce the dollar effect of a price move. It generally leaves less room to absorb losses, subject to the venue's margin rules.

For example, ten units entered at $200 have $2,000 of notional. A $5 adverse move produces a $50 price loss before costs, whether the position was supported by $1,000 or $400 of collateral. This assumes a linear payoff of one dollar per unit for each one-dollar price move.

A basic position-sizing formula

For that linear payoff, the starting calculation is:

Position quantity = planned price-loss budget ÷ distance between entry and intended exit.

For a long, an adverse exit is below entry. For a short, it is above entry. Use the positive distance between the two prices. CME's position-sizing lesson likewise connects the loss budget with stop distance and the monetary value of a price move.

The formula does not choose a suitable loss budget or exit level for you. Those are assumptions that need to be explicit before the result means anything. Moving an exit closer just to make a larger position fit changes the trade's behavior, not just its arithmetic.

Work through an example

Suppose a hypothetical account has $5,000 of equity. The trader sets a $50 planning budget for this trade, equal to 1% of that equity. The percentage is an illustration, not a recommended allocation. Entry is $200 and the intended adverse exit is $195.

That calculation spends the entire budget on price movement. It leaves nothing for trading fees, slippage, or any funding paid while holding a perpetual position.

Now assume the trader sets aside $10 of the same $50 budget for those costs and execution uncertainty. The price-loss budget becomes $40, producing eight units: $40 divided by $5. Entry notional becomes $1,600.

The $10 allowance is an assumption, not a guarantee or a quoted N1 charge. If the eight units exit at $193 instead, the price loss alone is $56. Fees and any net funding paid would add to that. A planned $50 loss has therefore not created a hard $50 loss ceiling.

Wider exits imply smaller positions for the same budget

Keeping a $40 price-loss budget and the same $200 entry, an intended exit at $190 gives a $10 distance. The calculated size becomes four units. An exit at $198 gives a $2 distance and twenty units.

The second calculation creates much more exposure. A closer exit might also be triggered by ordinary fluctuations, and a gap through it can create a larger-than-planned loss. Compare realistic exit and execution assumptions before treating the calculated size as usable.

Contract specifications matter too. For a contract with a fixed dollar value per tick, calculate loss per contract as stop distance in ticks multiplied by tick value. Divide the loss budget by that amount. Inverse and other nonlinear contracts need their own payoff formula. Round down to a permitted quantity increment rather than rounding above the budget.

Leverage is a constraint, not a sizing target

Available buying power tells you what may be permitted, not what fits a loss budget. After calculating a quantity, check whether the account can support it through the intended adverse move.

On N1, margin requirements depend on market parameters and account exposure, including possible exposure from open orders. Do not infer an exact liquidation price by simply dividing entry price by a leverage number. How liquidation works explains the separate maintenance-margin constraint.

Stops and other positions can change the outcome

N1's take-profit and stop-loss orders trigger from the index price. Activation attempts to reduce the position; it does not guarantee a fill. Available liquidity, a specified limit price, and market state still apply. The trigger reference is therefore not a promised execution price.

Also consider the account as a whole. Two trades that each appear to risk $50 can lose together when their prices move together. Resting entry orders can add exposure later, while funding and losses elsewhere can change available collateral. Cross and isolated margin explains how collateral boundaries affect that exposure.

Before submitting, check the quantity, notional, contract units, estimated costs, and remaining margin. Recalculate after material changes to entry, exit, account equity, or other positions. Position sizing is useful because it makes those assumptions visible; it cannot make uncertain execution certain.