Position size calculator
Trade size from the risk you allow per account and the distance to the stop.
Fill in the fields and the result will appear here automatically.
Derives size not from the amount you want to commit but from the amount you can afford to lose: however much is lost on one unit down to the stop, that is how many times the permitted risk fits into it. The position value comes out as a by-product and may well exceed the account — a consequence of the chosen risk and stop distance, without checking financing availability, and the share of the account is shown on its own row precisely so that this is visible. Fractional size is shown separately; the allowed lot step depends on the instrument and broker. If whole units are required, size rounds down so the modeled loss to the selected stop price does not exceed the amount set.
How it works
Formula and logic
With account D and chosen risk p%, risk budget R=D×p/100. Entry E and stop S must have positive distance a=|E−S|. Fractional size q=R/a; whole units floor(q), fractional position value q×E and account share 100×q×E/D. This assumes a linear payoff per unit without a contract multiplier, fees or slippage. It models a loss at the entered execution price and does not guarantee stop execution.
Example
An account of 100000, risk 1%, entry 250 and stop 240 gives 100 units worth 25000. The modeled loss at execution price 240 is 1000 before costs. For risk budget 100, entry 10 and stop 7, fractional size is 33.333…; 33 whole units give modeled loss 99. Rounding up to 34 would exceed the budget with loss 102.
Fields and units
- Account balance — $
- Risk allowed per trade, % — unitless
- Entry price — $
- Stop price — $
How to use
- — Enter the whole account balance rather than the free margin.
- — Set your own risk per trade above 0 and up to 100%; the calculator does not choose a safe percentage.
- — Enter the entry price and the stop price.
- — Compare position value with capital and the permitted lot step; execution eligibility is not checked.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- Investor.gov, US SEC, 18 August 2026: stop price is not guaranteed; stop-limit may remain unfilled
- Limitation
- Calculation assumes linear loss per unit and execution at the stated price. A stop guarantees neither price nor execution; margin, contract multiplier, lot step and costs need separate checks.
FAQ
Why is size derived from the stop rather than the amount invested?
It links a chosen money budget to the price difference per unit. This is a conditional loss at the stop execution price; gaps and instrument terms may produce a different outcome.
The position is worth more than the account — is that an error?
A small stop distance can produce this arithmetically. A value above the account does not establish financing availability or whether the stop suits the instrument; capital, margin and lot steps need separate checks.
Why do whole units round down?
If whole units are required, floor(q) does not exceed fractional size. At execution at the entered stop price this keeps modeled loss within the budget; rounding up could exceed it. Permitted steps and actual execution require separate checks.
Are fees and slippage included?
No. Fees and adverse execution can take the loss beyond the chosen budget; this model does not estimate the difference. A stop-limit order can also remain unfilled, unlike assuming execution at the entered price.
What risk percentage is considered sensible?
The model does not determine a suitable risk. It depends on the instrument, capital, related positions and loss probabilities. The percentage is a scenario input rather than a trade safety assessment.