Position size calculator: risk, units and notional

What the four inputs mean

Reference balance ($): the account amount used as the calculation base. Scenario loss fraction (%): the portion you enter as a loss allowance; enter 1 for 1%, not 0.01. The calculator does not choose this percentage for you.

Reference price A ($): your hypothetical entry per unit. Reference price B ($): your hypothetical stop per unit. Use the same currency and asset for both. For a long, a loss stop is below entry; for a short, above. The calculator uses absolute distance and has no direction selector.

Formula and output units

Loss allowance = balance × risk percentage ÷ 100. Units = loss allowance ÷ |entry − stop|. Notional = units × entry. Notional / balance (%) = notional ÷ balance × 100.

Dollar allowance divided by dollars per asset unit gives asset units: BTC when prices are USD per BTC, ETH when they are USD per ETH. It is not a number of exchange contracts; those require the venue’s contract multiplier and order-size rounding. The screen rounds units to six decimal places, so use the unrounded formula when checking the dollar result.

Worked examples

Hypothetical inputs: balance $1,000; risk 1%; entry $60,000; stop $58,800. Allowance = $10; distance = $1,200; units = $10 ÷ $1,200 = 0.008333333… BTC. Displayed units round to 0.008333; notional is $500 and notional/balance is 50.0%. A fill exactly at the stop loses $10 before costs.

Hypothetical short: balance $5,000; risk 2%; entry $3,000; stop $3,100. Allowance = $100; distance = $100; units = 1 ETH; notional = $3,000; notional/balance = 60.0%. The same arithmetic works with a stop above entry, but the calculator does not validate trade direction.

Notional is not margin

Notional measures market exposure. Initial margin is collateral supporting that exposure. In a simplified 5× example, $500 notional requires $100 initial margin: $500 ÷ 5. Changing leverage does not change this calculator’s units or the $10 entry-to-stop loss; leverage is not an input. Maintenance tiers, fees and collateral rules can change actual margin needs.

Use the margin calculator for collateral scenarios and the liquidation calculator for an approximate isolated-margin boundary. A stop price and a liquidation price are different concepts.

Common questions

Why does a closer stop produce more units? With the first example’s stop at $59,400, distance halves to $600, units double to 0.016666667… BTC and notional doubles to $1,000. This changes exposure, not the $10 allowance.

Does the allowance include costs? No. Fees, funding, spread and slippage are excluded; gaps or a worse stop fill can make losses larger. Estimate costs in the effective trading cost calculator.

Why is the result blank? Balance and prices must be positive; risk must be above 0% and no more than 100%; entry and stop must differ. The output does not check whether your stop is sensible or executable.