What you will learn
- Understand why position sizing is a matter of survival
- Apply the risk-per-trade rule
- Calculate a position size from capital, risk limit, and stop distance
- Connect sizing to the loss-recovery asymmetry
Everyone focuses on what to buy. Far fewer think carefully about how much to buy, and yet that second question decides whether you survive. Position sizing is choosing how much capital to put into any single investment, and it is the difference between staying in the game and being wiped out. A brilliant strategy sized too large can still ruin you.
The ultimate guide to position sizing (Desire To Trade)
Explains how much to risk per trade and how to size accordingly. Focus on the risk-per-trade rule.
Why size matters more than the pick
Even an investment with a genuine edge, a positive expected value, will lose sometimes. If any single position is large enough, one bad outcome can inflict damage that is hard or impossible to recover from. Position sizing determines how much any individual loss can hurt you, and therefore whether you can survive the inevitable string of losses that even a good strategy produces. The goal is not to maximize the gain on any one trade, but to ensure that no single trade can take you out.
The risk-per-trade rule
A widely used principle is to risk only a small, fixed fraction of your total capital on any one position, often 1 or 2 percent. Risking 1 percent means sizing the position so that if it falls to its predefined stop-loss, the loss is only 1 percent of the portfolio. With such a rule, no single trade can do serious damage, and even an unlucky run of losses leaves most of your capital intact. The position size follows from three things: your total capital, the fraction you will risk, and how far the stop-loss sits from your entry.
Key terms
- Risk per trade
- The fraction of total capital you are willing to lose on a single position, often 1 to 2 percent.
- Stop-loss
- A predefined exit price that caps the loss on a position.
- Position size
- How many shares or how much capital to commit, chosen so the loss at the stop equals your risk limit.
Sizing a position
You have 50,000 dollars and will risk 1 percent on a trade. You plan to buy a stock at 50 dollars and set a stop-loss at 48 dollars. How many shares should you buy?
- Dollar risk allowed. 1 percent of 50,000 is 500 dollars.
- Risk per share. The stop is 2 dollars below the entry, so each share risks 2 dollars.
- Divide. 500 dollars of allowed risk divided by 2 dollars per share is 250 shares.
Why it matters: Even though the position is 12,500 dollars, a quarter of your capital, you only risk 500 dollars, because the stop is close. Position size is set by the risk to the stop, not by the position's dollar value.
Calculate a position size
You have 100,000 dollars and will risk 2 percent. You buy at 20 dollars with a stop-loss at 19 dollars. How many shares should you buy?
What to buy is the glamorous question. How much to buy is the one that decides whether you survive. Size for survival first.
How to calculate your position size and risk per trade (Derrick the Trader)
A worked walkthrough of the same calculation. Follow along to lock in the method.
The asymmetry that makes this urgent
Position sizing matters so much because of a hard mathematical fact you will study fully in the next lesson: gains and losses are not symmetric. Recovering from a loss requires a larger percentage gain than the loss itself. A 50 percent loss requires a 100 percent gain just to break even, and deeper losses require recoveries that border on impossible. Because large losses are so much harder to undo, avoiding them through careful sizing matters more than squeezing out extra gains. This is also why concentration is dangerous: letting one position dominate means a single bad outcome can be catastrophic.
Survival enables compounding
The core principle is that you cannot compound your wealth if you are wiped out. Staying in the game is the precondition for every future gain, and a single oversized bet gone wrong can end the game permanently. This echoes the risk-of-ruin reasoning from Unit 5: a positive-expected-value strategy can still ruin you if you bet too much, because the path to that favorable average runs through losses your sizing must be able to absorb. The Kelly criterion later in this unit makes optimal sizing mathematically precise, but the core lesson is already clear: size so that no single loss, and no plausible run of losses, can take you out.
Which trader survives a losing streak?
Trader A risks 2 percent of capital per trade, while Trader B risks 25 percent per trade. Both hit an unlucky run of five losses in a row. Who is more likely to survive and keep investing?
Trader A risks only 2 percent per trade, so five losses leave roughly 90 percent of capital intact. Trader B risks 25 percent each time and is down over 75 percent after five losses, facing a near-impossible recovery. Small position sizes are what let you survive losing streaks.Why size for survival?
In your own words, explain why sizing positions to survive losses matters more than sizing them to maximize gains.
Write an answer before comparing it with the model response.
Model answer
Even a good strategy loses sometimes, and losses hurt more than equivalent gains help, because recovering from a big loss takes a disproportionately large gain. If I size positions too large chasing maximum gains, one bad outcome or an unlucky streak can inflict a loss I cannot recover from, ending my ability to invest at all. But if I risk only a small fixed fraction on each position, no single loss can do serious damage, so I stay in the game. Since I can only compound my wealth if I survive, protecting against ruin comes first, and maximizing any one trade's gain comes a distant second.
Position sizing is really about controlling losses. The next lesson looks directly at losses themselves, the drawdown, and the harsh arithmetic of digging out of a hole.