Position sizing with a fixed % risk

Most traders spend hours on where to buy and almost no time on how much to buy. Yet the size of a position decides how much a wrong idea costs, and every trader is wrong often. Fixed-percentage risk sizing is a simple way to make that cost predictable. You decide, before the trade, what share of your account you are willing to lose if the trade hits your stop, and the share count falls out of the math.

This article explains the method in plain English, works through an example, and covers the places where it breaks. It is education only. It doesn't tell you what to trade, what limits to choose, or whether to trade at all.

The idea in one sentence

Pick a fixed percentage of your account to risk on each trade, measure the distance from your entry to your stop, and buy only as many shares as that distance allows.

The formula

1. Risk budget = account size × risk per trade (%). With a $10,000 account and 1% risk, the budget is $100.
2. Risk per share = entry price − stop price (for a long trade). Buy at $50 with a stop at $48 and each share risks $2.
3. Shares by risk = risk budget ÷ risk per share, rounded down. $100 ÷ $2 = 50 shares.

For a short trade the distance is stop minus entry. The logic is the same: the further away the stop, the fewer shares you can hold for the same dollar risk.

Why you also need a position cap

The formula has a blind spot. A very tight stop makes the risk per share tiny, so the share count balloons. A $0.10 stop on a $50 stock would allow 1,000 shares, or $50,000 of stock in a $10,000 account. That isn't possible in a cash account and it isn't sensible in a margin account, because a gap through the stop can cost far more than the plan.

A position cap fixes this. It limits any one position to a share of the account, for example 10%. The cap gives its own share count:

• Shares by cap = account × cap % ÷ entry price, rounded down. $10,000 × 10% ÷ $50 = 20 shares.
• Final shares = the smaller of shares by risk and shares by cap.

The worked example

Here is the whole calculation with fictional numbers:

Notice that the final risk, $40, is less than the $100 budget. That's normal when the cap binds. The budget is a ceiling, not a target. If you want the full $100 of risk, the honest options are a wider stop that still makes sense for the setup, or a higher cap that you're comfortable with. Changing the numbers just to make the trade bigger defeats the purpose.

Adding a daily loss limit (the "fuse")

Per-trade risk controls one trade. It doesn't stop a bad day from turning into five bad trades. A daily loss limit does. Set it once, while you're calm: for example 2% of the account, or $200 in the example. With $40 at risk per trade, that's five full-stop losses before you are done for the day.

A simple rule set many traders write down: at 75% of the daily limit used, slow down and only take the cleanest setups. At 100%, stop trading for the day. The point isn't the exact numbers. It's that the decision was made before emotions were involved.

You can try both calculations in the browser with our free position size and daily loss calculator, or download the free Risk-Fuse Calculator spreadsheet for Excel and Google Sheets.

Where fixed % sizing breaks

• Gaps and slippage. A stop is an instruction, not a guarantee. If a stock gaps below your stop overnight, the fill can be far worse, and the loss can exceed the plan.
• Fees and spreads. On small positions, commissions, fees and the bid-ask spread add to the real risk. Include them if they matter for your account.
• Correlated positions. Five positions in the same sector, each risking 1%, can behave like one 5% position on a bad day. Some traders also cap total open risk.
• Liquidity. Thinly traded stocks can make it hard to exit at the stop. A smaller size is the usual answer.
• Moving the stop. The method only works if the stop you size from is the stop you honor. Widening it after entry quietly changes the risk you signed up for.

Choosing your own numbers

There is no correct risk percentage for everyone. People choose based on account size, experience, how often they trade and how they react to losing streaks. A useful exercise is to ask: if I lost my per-trade amount ten times in a row, would I still follow my plan? A losing streak of that length is ordinary for many styles of trading. If the answer is no, the number is probably too high for you. This is a planning question, not a recommendation; your situation is your own.

Keep a record

Write down the size calculation with every trade: entry, stop, shares, dollars at risk and which limit decided the size. Over a few weeks the record shows whether you are sizing consistently or drifting when you feel confident. That record is often more useful than any single trade.

More on this topic: the Journal & Risk category lists every related tool, including what’s coming soon. Or browse all 7 categories.