What does position size actually decide?
Two traders take the identical signal, on the identical chart, at the identical moment. One finishes the year up; the other empties the account. Nothing about the analysis differed. The only variable was how much each one put on.
That is the uncomfortable centre of this course. Charts, indicators and patterns decide whether a trade wins. Position size decides how much each answer matters — and therefore whether a run of wrong answers is survivable.
The two numbers people confuse
- Position size (exposure) — the money committed. Buy 200 shares at $50 and your exposure is $10,000.
- Risk — the money that actually leaves if the trade goes wrong. If you would exit at $46, the risk is $4 per share × 200 shares = $800, not $10,000.
Same trade, two completely different numbers. Beginners track the first and never compute the second. Everything in the next three units is built on the second.
Why the size number dominates
Take a hypothetical system that wins half its trades and pays 2× the amount risked on winners. It has a genuine mathematical edge (Unit 3 shows exactly why). Now push four consecutive losses through it at different sizes — and four in a row is ordinary, not exotic:
| Risked per trade | Equity after 4 losses (from $100,000) | Drawdown | Gain needed to get back |
|---|---|---|---|
| 1% | $96,060 | 3.9% | 4.1% |
| 5% | $81,451 | 18.5% | 22.8% |
| 25% | $31,641 | 68.4% | 216% |
Check the bottom row yourself: 0.75 × 0.75 × 0.75 × 0.75 = 0.3164. Two thirds of the account is gone, and the same edge now has to more than triple the remainder just to reach flat. The edge never changed. The size did.
Risk of ruin, in one sentence
There is a whole literature computing the probability of ruin from win rate, payoff and bet fraction. You don't need the formulas yet to absorb what all of them share: as the fraction risked per trade rises, the probability of eventual ruin rises far faster than the fraction does — and past some point even a positive-edge system is mathematically doomed. Size doesn't amplify an edge; past a point it converts one into a countdown.
This course never tells you what fraction to use. It shows you what each fraction does to the arithmetic, so the choice is yours and informed rather than yours and accidental.
Try it now
- Take a hypothetical $25,000 account. Compute the equity remaining after five consecutive losses at 1%, at 3% and at 10% per trade (multiply by 0.99, 0.97 and 0.90 five times).
- For each ending balance, compute the percentage gain needed to return to $25,000:
drawdown ÷ remaining balance. - Now ground it on a price you did not invent. Take the latest close from the month of real sessions below and write down both numbers for a hypothetical 100-share position: the exposure, and the risk if you exited 5% below entry. Notice how far apart they are.
Next: the sizing rule that makes the fraction automatic.