What position sizing actually means
Position sizing is the decision of how many shares or contracts to buy. It is not the same as picking a stock or timing an entry. You can be right about direction and still go broke if the size is wrong, because one oversized loss can erase a month of good trades. Sizing is the one input that controls how much a single bad trade is allowed to cost you.
The professional way to think about it flips the usual question. Instead of asking “how much do I want to make,” you ask “how much am I willing to lose if I am wrong.” You fix that loss first, then let it dictate the size.
The 1% rule, explained
The 1% rule says no single trade should risk more than 1% of your account. On a $10,000 account, that is $100 of risk per trade. Risk is the distance from your entry to your stop, multiplied by your size, not the total dollars you put in. Many traders use anywhere from 0.5% to 2%; 1% is the common starting point because it lets you lose ten trades in a row and still have roughly 90% of your account intact.
The point is not the exact percentage. The point is that you decide the number before the trade, and the number is small enough that no single loss threatens your ability to keep trading.
The formula, in three steps
- 1. Risk amount = account size × risk percent. Example: $10,000 × 1% = $100.
- 2. Risk per share = entry price − stop price. Example: $100 − $98 = $2.
- 3. Share count = risk amount ÷ risk per share. Example: $100 ÷ $2 = 50 shares.
Notice what happened: you never picked a dollar amount for the position. The size came out of the risk. Those 50 shares at $100 is a $5,000 position, but that $5,000 is a result, not a decision. Tighten the stop to $99 and the same $100 of risk buys you 100 shares; widen it to $95 and it buys you 20. The risk stays fixed at $100 no matter what.
A worked example
Say you have a $25,000 account and you trade a 0.75% risk budget, so $187.50 per trade. You want to buy a stock at $42.00 with a stop at $40.50, a risk of $1.50 per share. Your size is $187.50 ÷ $1.50 = 125 shares, a $5,250 position. If the stop hits, you lose $187.50, exactly the number you chose up front. If the stock runs to $48, you make $750 on the same 125 shares. Same rule, every trade, no guessing.
Why size off risk, not conviction
The reason this works is that it removes emotion from the one decision emotion ruins most. When you size off how sure you feel, you put the most money on the trades where you are most likely to be blinded, and the least on the boring, high-probability setups that actually pay the bills. Fixed-fractional sizing does the opposite: it keeps every loss the same size, so a losing streak is survivable and a winning streak compounds cleanly.