The one formula behind every position size
Position size = Money you are willing to lose ÷ Loss per unit if your stop is hit
Everything else is just finding the loss per unit. For a stock, it is the distance from entry to stop. For futures, it is the stop distance in ticks times the tick value. For forex, it is the stop in pips times the pip value per lot.
Futures example
On a $50,000 account risking 1% ($500) with an 8-point stop on ES: each contract risks 8 points × $50 = $400, so you can trade 1 contract (actual risk $400). Trade MES and each contract risks $40, so 12 contracts keeps you at $480.
How much should you risk per trade?
Many traders keep risk between 0.25% and 1% of the account per trade, because a run of losses is normal even with a profitable strategy. The risk of ruin calculator shows how fast a bigger number can end an account. In a prop-firm evaluation, size against the drawdown limit, not the account size — see the prop firm challenge calculator.
Round down, always
The calculator rounds down to whole contracts and shares. If one contract already risks too much, the honest answer is a smaller contract or no trade.
FAQ
How do I calculate position size?
Divide the dollar amount you are willing to lose by the loss per unit at your stop. For futures that is stop ticks × tick value per contract.
What percentage should I risk per trade?
Commonly 0.25%–1% of the account. Lower risk keeps a normal losing streak survivable.
Should I use micros or minis?
If one mini contract risks more than your limit, micros (1/10 the size) let you size precisely instead of oversizing.