Position sizing is how much of your account you put into a single trade, chosen so that a stopped-out loss costs you a fixed, survivable amount. It is the risk layer of any ETF trend trading system, and it matters more than the entry signal: a great signal with terrible sizing still blows up an account.
The most common method is risk-based sizing. You decide the percentage of your account you are willing to lose if your stop is hit, then buy just enough shares so that the loss equals that amount. Enter your numbers below and the tool does the rest.
How the calculation works
The formula is simple: shares = (account size × risk %) ÷ (entry − stop) for a long position.
Example: a $10,000 account, risking 1% ($100), buying an ETF at $100 with a stop at $95. Risk per share is $5, so you buy 20 shares. If the stop is hit, you lose 20 × $5 = $100, exactly 1% of the account.
Why this is the number that keeps you in the game
Trend systems lose small and win big. A losing streak is normal and expected. If each loss is capped at 1% of your account, ten straight losses cost you about 10% and you survive to catch the next trend. If you size every trade to the maximum, the same streak ends the account. Sizing is the difference between a losing streak being an inconvenience and being a catastrophe.
Use it with a stop rule
Risk-based sizing only works if you actually have a stop. Many trend traders use a rule like the 7% to 8% stop: sell if a position falls that far below your entry. Pair a defined stop with the sizing above and your downside is capped before you ever place the order. See the risks guide for the other things that can go wrong.
FAQ
How do I calculate position size for a trade?
Divide your dollar risk by the risk per share. Dollar risk is your account size multiplied by the risk percentage you are willing to lose. Risk per share is the entry price minus the stop-loss price for a long position. The result is the number of shares to buy.
What risk percentage should I use?
Many traders risk 1% or less of their account on a single trade, and 2% is generally considered aggressive. The right number is the loss you can take repeatedly without abandoning your system, so start small.
Why does position sizing matter in trend trading?
Trend systems win by cutting losses small and letting winners run. Correct sizing keeps a normal losing streak survivable, while over-sizing turns that same streak into account damage.
Can I buy fractional ETF shares?
Many brokers now allow fractional shares, but some exchanges still settle in whole units. Round down to whole units if your broker does not support fractions.
Disclaimer: Educational tool only, not financial advice. It performs arithmetic on your inputs; it does not recommend any trade, position, or strategy. Full disclaimer and affiliate disclosure.
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