CluenexWhat is position sizing?
Position sizing is deciding how much money to put into one investment. The most common method works backward from the loss you can accept: pick a maximum dollar loss, then buy only as many shares as keep a stop-out within that loss.
Without a method, people buy round numbers like 100 shares. That produces very different risk on different stocks. 100 shares of a $10 stock puts $1,000 at work, and 100 shares of a $500 stock puts $50,000 at work.
What is the position sizing formula?
The formula is: shares = (account value × risk percent) ÷ (entry price − stop price). The top is the dollars you accept losing. The bottom is how much you lose per share if the stop is hit.
Worked example with a $50,000 account:
- Account risk: $50,000 × 1% = $500.
- Risk per share: entry $100, stop $95, so $100 − $95 = $5.
- Shares: $500 ÷ $5 = 100 shares.
- Dollars invested: 100 × $100 = $10,000, which is 20% of the account.
Step 4 matters. The position is a fifth of the account, yet the planned loss is only 1%.
| Account | 1% risk | Stop distance | Shares | Position value | Share of account |
|---|---|---|---|---|---|
| $10,000 | $100 | $5 | 20 | $2,000 | 20% |
| $50,000 | $500 | $5 | 100 | $10,000 | 20% |
| $100,000 | $1,000 | $10 | 100 | $10,000 | 10% |
A wider stop gives fewer shares. A tighter stop gives more shares, but a stop that is too tight gets hit by normal price noise.
How much of my account should I risk per trade?
A widely used guideline is 1% of the account per trade, with 2% as an upper bound for experienced people. This is a convention, not a law, and no regulator sets it.
The arithmetic shows why small numbers help. Losses compound against a shrinking balance, so the drawdown after a streak of losing trades is below the simple sum. After 10 straight losses, the account is down about 9.6% at 1% risk (1 − 0.99^10), 18.3% at 2% (1 − 0.98^10) and 40.1% at 5% (1 − 0.95^10).
Losing streaks happen even to sound strategies. The smaller the risk per trade, the more survivable they are. A deeper explanation of recovery math is in What is a Drawdown.
How do I set the stop distance?
The stop should sit where your reason for the trade is proven wrong, not where it makes the share count look nice. Common choices are below a recent swing low or a multiple of the stock's average true range.
Average true range (ATR) is the typical daily price movement over a period, often 14 days. A stop of 2 × ATR below entry gives normal swings room to happen.
Illustration with made-up numbers: entry $250, ATR $15, stop at $250 − (2 × $15) = $220. Risk per share is $30. With $500 of account risk, shares = $500 ÷ $30 = 16 (rounded down), a $4,000 position or 8% of a $50,000 account.
See How to Set Stop Losses for the methods in more detail.
What is the Kelly Criterion, and should I use it?
The Kelly Criterion is a formula for the bet size that maximizes long-run growth if you know your odds exactly. For a simple win or lose bet it is: Kelly % = W − (1 − W) ÷ R, where W is the win probability and R is average win divided by average loss. It was published by John Kelly in 1956.
Example: win rate 55%, average win $500, average loss $300, so R = 1.67. Kelly % = 0.55 − 0.45 ÷ 1.67 = 0.55 − 0.27 = 28%.
That number is a warning as much as a guide. Kelly assumes the win rate and payoffs are known and stable. In real markets they are estimates, and overestimating your edge makes full Kelly dangerous. Many practitioners use a fraction of Kelly, such as half, for this reason.
For most people with a regular job and a brokerage account, a fixed 1% rule is simpler and safer than Kelly.
Do stops always limit the loss to my planned amount?
No. A stop order becomes a market order once triggered, so the fill can be worse than the stop price. If a stock gaps down overnight on bad news, you can lose far more than 1%. The SEC's investor education site explains that a stop price does not guarantee the sale price.
This is why a second rule helps: cap the size of any single holding. Say the cap is 10% of a $50,000 account, which is $5,000. At $500 a share, that allows at most 10 shares, even if the stop-based formula gave 30.
Related: Concentrated Stock Positions.
Where does position sizing fall short?
The method controls loss per trade, not whether the trade is a good idea. A well-sized bad idea still loses money.
Common mistakes:
- Buying round lots. Share count should come from the formula, not habit.
- Choosing shares first, then a stop. This hides how much you really risk.
- Risking more on high conviction. Feeling sure is not evidence of an edge.
- Ignoring correlation. Five positions that all fall together on one news item act like one big position. See How to Diversify a Stock Portfolio.
- Ignoring costs and taxes. Frequent stop-outs can create trading costs and taxable losses and gains.
The method also fits poorly for long-term index investors who never use stops. For them, sizing means deciding what percent of the portfolio each holding gets, which is a different question covered in Position sizing for long-term holdings.
What this means for you
If you invest your own money from a paycheck, keep the process short and repeatable.
- Pick a risk percent, such as 0.5% to 1%, and write it down.
- Before buying, write the price at which your idea is wrong. That is your stop or review point.
- Compute shares with the formula and round down.
- Check the position value as a percent of your account. If it is above your single-holding cap, buy less.
- Add up the risk across all open positions. Ten positions at 1% each is 10% of the account at risk if they fall together.
If you want context on a company before you decide how much to own, Cluenex stock pages show business quality, the main risk and insider activity in plain English. Charts there include indicators such as moving averages, which some people use to place stops, but Cluenex treats chart patterns as context rather than signals. Start at cluenex.com/glossary/ to see how each number is explained.