TL;DRDetermining how many contracts to trade on each setup based on your account size, risk tolerance, and the distance to your stop loss. This is the skill that prevents a single bad trade from doing catastrophic damage.
Position sizing is the process of deciding how many contracts to trade. It's the bridge between having a trade idea and executing it with the right amount of risk.
A good trade setup with the wrong position size becomes a bad trade. If you risk too much, one loss can set you back weeks. If you risk too little, your winners don't make a meaningful difference.
The core position sizing calculation is: Number of contracts = (Account risk in dollars) / (Stop distance in dollars per contract).
Account risk is the maximum dollar amount you're willing to lose on a single trade. Most traders set this at 1-2% of their account. On a $25,000 account, 1% risk means you're willing to lose $250 per trade.
Stop distance is how far your stop loss is from your entry, converted to dollars. If you're trading ES with a 10-point stop, that's $500 per contract (10 points x $50). With $250 of risk and a $500-per-contract stop, you'd trade 0.5 contracts. Since you can't trade half a contract on ES, you'd either skip the trade or use micro contracts.
Risking a fixed percentage of your account on every trade creates a natural scaling mechanism. When you're winning, your account grows and your position sizes grow with it. When you're losing, your account shrinks and your position sizes shrink automatically, slowing the bleeding.
This is the opposite of what most losing traders do. Losing traders tend to increase size after losses (trying to make it back) and decrease size after wins (becoming cautious). Percentage-based sizing does the reverse, which is mathematically correct.
The standard recommendation is 1% risk per trade for new traders and up to 2% for experienced traders with proven track records.
Prop firms add an extra constraint: your total drawdown is limited. This means your position sizing needs to account for not just individual trade risk but cumulative risk across a series of trades.
If your prop firm allows a $2,500 maximum drawdown, risking $250 per trade gives you room for 10 consecutive losers before you're out. Risking $500 per trade gives you only 5.
Many traders who pass prop firm evaluations use even smaller risk per trade during the early phase, increasing size only after building a profit cushion.
Basic position sizing on ES
Account: $50,000. Risk per trade: 1% ($500). You see an ES setup with a 10-point stop loss.
10 points on ES = $500 per contract. $500 risk / $500 per contract = 1 contract. If your stop were 5 points ($250 per contract), you could trade 2 contracts. If your stop were 20 points ($1,000 per contract), the math says 0.5, so you'd use 5 MES contracts instead.
Prop firm sizing
You're in a $50,000 prop firm evaluation with a $2,500 trailing drawdown.
$2,500 / 10 trades = $250 max risk per trade. On ES with a 5-point stop ($250/contract), that's 1 contract. Start conservative, increase size only after building a profit buffer.
Sizing based on how much margin you have instead of how much you can afford to lose
Margin availability and risk tolerance are different things. Trade the calculated size, not the maximum allowed.
Using the same number of contracts on every trade regardless of stop distance
A 5-point stop and a 20-point stop have very different risk profiles. Always adjust contract count based on stop distance.
Increasing size after a losing streak to recover faster
This is the fastest path to blowing up. Percentage-based sizing naturally reduces your size during drawdowns, which is the correct behavior.