Explainer · Risk Management
Position sizing: decide the loss before the trade
Position sizing turns one decision, how much you are willing to lose on a trade, into how many units to buy or sell. It is the simplest risk control a beginner has, and the one most often skipped.
Quick answer
Position sizing means choosing how many units to trade so that, if your stop is hit, you lose a fixed amount you decided in advance. Divide that amount by the distance between your entry and your stop. Gaps can still make the real loss larger [4].
Key points
- Start from the money you can afford to lose, which regulators describe as risk capital [1].
- Units = amount at risk divided by the distance from entry to stop.
- A wider stop means fewer units for the same risk; a tighter stop means more units, not less risk.
- A stop becomes a market order when triggered, so the real loss can exceed the plan [5].
- Small losses compound: ten 5% losses in a row cost about 40% of an account (calculated).
On this page
What is position sizing?#
Position sizing is the step between deciding to trade and placing the order. Instead of asking "how much do I want to buy?", you ask "how much am I prepared to lose if I am wrong?" and work backwards to the number of units.
That order matters. Regulators repeat one rule above all others for people trading leveraged products: use only money you can afford to lose. The National Futures Association calls this risk capital, meaning money over and above what you need for necessities, emergencies, savings and long-term goals [1]. The SEC's guidance for day traders says the same and adds that it should never be money for living expenses, retirement or student loans [2].
How do you calculate a position size?#
You need three numbers before you open the trade: your account size, the share of it you are willing to lose on this one idea, and the price where you will admit you were wrong (your stop). The distance between your entry and that stop is your risk per unit.
units = (account size × risk %) / (entry price - stop price)
- Decide the amount at risk
Account $10,000 × 1% = $100. This is the most you plan to lose if the stop fills at its price.
- Measure the distance to the stop
Entry $100, stop $95: each unit loses $5 if the stop fills at $95.
- Divide
$100 / $5 = 20 units. The position is worth 20 × $100 = $2,000, or 20% of the account.
- Check the result
Ask whether the position value and any margin needed fit your account and your broker's rules before you place the order.
Why does the distance to the stop change the size?#
Because the amount at risk is fixed, the stop distance is the only lever left. A wide stop gives the trade room but forces a small position. A tight stop allows a large position but leaves little room before you are taken out.
Neither is safer on its own. What matters is that the stop sits where your reason for the trade would be proven wrong, and the size follows from it, not the other way round.
| Stop price | Distance per unit | Units | Position value | Share of account |
|---|---|---|---|---|
| $98 | $2 | 50 | $5,000 | 50% |
| $95 | $5 | 20 | $2,000 | 20% |
| $90 | $10 | 10 | $1,000 | 10% |
| $80 | $20 | 5 | $500 | 5% |
Every row loses $100 if the stop fills at its price. Gaps and slippage can make the real loss larger.
What does a losing streak do to different risk levels?#
Losing streaks happen to everyone, and losses compound: each loss is taken from a smaller account. The table shows ten losses in a row at three risk levels. The recovery column uses the arithmetic from our page on why a 50% loss needs a 100% gain: gains needed = loss / (1 - loss) [3].
| Risk per trade | Account after 10 losses | Total drawdown | Gain needed to recover |
|---|---|---|---|
| 1% | $9,043.82 | 9.56% | 10.57% |
| 2% | $8,170.73 | 18.29% | 22.39% |
| 5% | $5,987.37 | 40.13% | 67.02% |
Research on downside risk found that many people misjudge this arithmetic: in one experiment with 981 US adults, 50.8% answered both downside-risk questions wrongly [3]. Writing the numbers down before you trade is a cheap way not to be one of them.
Can you lose more than you planned?#
Yes. The plan assumes your stop fills at its price, and it often will not. The SEC explains that when the stop price is reached, a stop order becomes a market order, and the price you get can deviate significantly from the stop [4]. FINRA's example: with a $50 floor, a fast market can fill you at significantly less than $50 per share [5].
In our example, if the price gapped from $96 straight to $90, the 20-unit position would lose $200 instead of $100, which is 2% of the account rather than 1% (calculated).
Does position sizing make trading safe?#
No. It controls how much one wrong trade costs; it does not make trades right. Official data on leveraged retail trading is sobering: a French AMF study of 14,799 active individual investors found that 89% lost money over four years [7], and ESMA's 2018 decision cited separate national studies in which between 75% and 89% of retail clients lost money on CFDs [8].
Position sizing is how you stay in the game long enough to learn. It is not a strategy, and it cannot turn a losing approach into a winning one.
Mistakes beginners make with position sizing#
- Sizing by confidence
Buying more because a trade "feels right". Confidence does not change the distance to your stop.
- Moving the stop to fit the size
Choosing the size first and then squeezing the stop closer so the maths works. The stop should come from the trade idea; the size follows.
- Forgetting costs
Spreads, commissions and financing reduce every result. The SEC notes day traders pay large amounts in commissions and should know how much they need to make just to break even [2].
- Raising risk after losses
Doubling the size to win back a loss makes the next loss bigger. Keep the percentage fixed and let the size shrink with the account.
Frequently asked questions#
What percentage of my account should I risk per trade?
There is no official figure in the regulator guidance we cite [1]. Many traders choose a small, fixed percentage so that a run of losses cannot cripple the account. Whatever you pick, decide it before the trade and keep it the same.
Is position sizing the same as leverage?
No. Leverage is how much exposure you control per dollar of margin. Position sizing is how much exposure you choose to take. You can use a broker that offers high leverage and still size each trade so a stopped-out loss is small.
Does a stop-loss guarantee my maximum loss?
No. FINRA states that stop prices are not guaranteed execution prices [5]. In fast markets or price gaps the fill can be worse than your stop.
Should the size change as my account grows or shrinks?
If you risk a fixed percentage, yes. The amount at risk is a share of the current account, so it shrinks after losses and grows after gains.
The bottom line#
Decide the loss first, measure the distance to your stop, divide, and only then place the order. Use money you can afford to lose, remember that a stop is not a guaranteed price, and treat position sizing as a seatbelt, not an engine. Try your own numbers in the position size calculator, and read the risk disclosure before trading anything with leverage.
Sources
- Investor Best Practices | NFA.
- Day Trading: Your Dollars at Risk.
- Downside financial risk is misunderstood.
- Stop, Stop-Limit, and Trailing Stop Orders - Investor Bulletin.
- Stop Orders: Factors to Consider During Volatile Markets.
- Customer Advisory: Eight Things You Should Know Before Trading Forex.
- Perspectives on the techniques used to market speculative trading on the Forex and binary options markets.
- European Securities and Markets Authority Decision (EU) 2018/796 of 22 May 2018 to temporarily restrict contracts for differences in the Union in accordance with Article 40 of Regulation (EU) No 600/2014.
Education only. This page is not investment, tax or legal advice. Trading and crypto can lose you money. See our risk disclosure.
