What is Position Sizing? Definition, Formula, and Example
Position sizing is the process of determining how many shares or contracts to trade based on account size, acceptable risk per trade, and the distance to the stop loss.
What is Position Sizing?
Position sizing is the discipline of deciding how large a trade should be before entering it. It converts an abstract risk tolerance, usually expressed as a fixed percentage of account equity, into a concrete share or contract count using the distance between the entry price and the stop loss. Position sizing is the single largest determinant of long-run survival in trading: two traders with identical entries and exits produce radically different equity curves purely from sizing decisions.
How Position Size Is Calculated
The standard fixed-fractional formula is:
Shares = (Account Equity x Risk %) / (Entry Price − Stop Price)
The numerator is the dollar risk budget for the trade. The denominator is the dollar risk per share. Dividing the two gives the maximum position size that keeps the loss at or below budget if the stop is hit.
Variants include volatility-based sizing, where the stop distance is set as a multiple of average true range, and the Kelly criterion, which sizes positions from edge and odds: f* = p − (q / b), where p is win probability, q is loss probability, and b is the win-to-loss payoff ratio. Most practitioners use fractional Kelly, half or quarter, because full Kelly produces severe drawdowns.
Worked Example
A trader has a $50,000 account and risks 1% per trade, a $500 budget. They want to buy TSLA at $250 with a stop at $240, a $10 per-share risk. Position size = $500 / $10 = 50 shares. Dollar exposure is $12,500, or 25% of the account, but risk is exactly 1%. Now suppose the setup instead requires a wide stop at $230, a $20 risk. Size = $500 / $20 = 25 shares, exposure $6,250. The formula automatically shrinks the position as volatility or stop distance grows, which is the entire point: risk stays constant while size flexes.
When Traders Use It
Position sizing is applied before every trade, and it is the mechanism behind portfolio-level rules: max 1% risk per trade, max 5% total open risk, max 10% exposure per sector. Systematic traders backtest sizing rules as rigorously as entry signals, because sizing dominates the shape of drawdowns. Options traders apply the same logic by capping premium at risk per position. Tracking realized risk per trade over time in the trade journal reveals whether actual sizing matches the plan or drifts under emotion.
Limitations and Common Misconceptions
The formula assumes the stop fills at the stop price. Gaps, halts, and slippage can make the realized loss exceed the budget, so sizing for illiquid names requires a buffer. A common misconception is that position sizing creates edge; it does not. Sizing controls the path, not the expectation, and no sizing rule rescues a negative-expectancy system. Another error is sizing off the entry-to-target distance instead of entry-to-stop, which inflates size exactly when conviction is highest and losses are largest. Correlated positions are a third trap: five 1% risks in five semiconductor stocks are effectively one 5% risk.