Series disclaimer: Educational content only, not financial advice. The numbers here are illustrative risk-management math, not a promise of returns.
Part 3 introduced stops and survival. This part makes risk quantitative: not "I'll be careful" but "every trade risks exactly X% of my account, computed before entry." Position sizing is the single most-overhyped-then-ignored skill in trading because it is the only knob you control in a probabilistic game.
Risk per trade, not position size#
The mental shift everyone resists: you do not size by "how many shares/contracts I can afford." You size by how much money I am willing to lose on this trade.
Risk per trade = (entry − stop) in price × position size.
If entry is 9.50, the per-share risk is 50, then size = 0.50 = 100 shares. The stop defines the risk; the sizing is derived, never the other way around.
Fixed-fractional sizing: the 1% rule#
The standard baseline: risk a fixed fraction of the account per trade — most commonly 1%, large enough to matter, small enough to survive a bad streak.
| Trade streak | Account after 10 trades (1% each, worst case) |
|---|---|
| 10 losses in a row, full 1% loss each | ≈ 90.4% of starting capital |
Losing 100,000 account (1%) leaves 99%. Losing 10% once (a common "sizing up" temptation) leaves 90% — and now you need +11% just to be even. Loss asymmetry: a 50% drawdown requires a 100% gain to recover. Fixed-fractional sizing converts that asymmetry into controllable daily math.
Position size formula in practice#
Risk budget = Account × Risk % (e.g., $50,000 × 1% = $500)
Per-share risk = |Entry − Stop| (e.g., $10.00 − $9.50 = $0.50)
Position size = Risk budget ÷ Per-share risk (e.g., $500 ÷ $0.50 = 1,000 shares)
Never reduce the stop to "fit" a larger size. The stop is a market decision (support, structure); the formula follows. If the resulting size is too large, the trade is omitted, not resized.
What fractional sizing does to a luck-based series#
Every strategy has variance. Sizing is what determines whether variance is a bump or a bankruptcy.
Two traders, same edge. Trader A risks 1%, Trader B risks 10%. A run of 6 losses: A is down ≈5.9%; B is down ≈46.9%. B now needs close to +90% to be even. A barely notices.
The distribution of outcomes in markets is fat-tailed. The "it'll come back" illusion fails precisely when sizing has already eroded the equity curve. This is why risk management is framed as survival, not returns.
When leverage and options enter the picture#
Options and margin change notional exposure while your risk budget should stay the same.
- For a leveraged position, size by risk (stop distance × leverage), not by notional. If you'd risk 0.50 risk still costs $500.
- For options, the max loss is the premium: risk budget = premium × contract count. A 500 → 250 contracts of notional control is cheap-looking but exactly as risky as the $2 premium says.
- Time decay (theta) does not wait for your stop. Options budgets need to account for the trade expiring worthless even when direction is right.
The 2% from the emotional side#
The 1% rule is not about math alone; it's about judgment. A trade risking 1% cannot make you emotionally violent. The moment a single position can swing your month, decisions get defensive: stops moved, entries chased, conviction replaced by hope. Position sizing is a discipline device before it is a profit device.
Daily and weekly risk budgets#
Derived limits keep a bad day from becoming a bad month:
- Per-trade risk: 1% (fixed-fractional ±)
- Daily cap: stop after 2–3 full per-trade losses (e.g., 2–3%)
- Weekly cap: hard stop ~5%; force a pause to diagnose, not to revenge
The "revenge trade" is the signature of violated budgets. If the rule says stop, the market that day is done — regardless of the tempting setup.
Common sizing mistakes#
| Mistake | Correct frame |
|---|---|
| "Bought 1,000 shares because I like it" | Sized from entry − stop × 1% risk |
| Tight stop to afford size | Stop is a market decision; size follows |
| Hitting max loss, then doubling in | Daily/weekly caps exist precisely for this |
| Sizing after entry | Formula runs before the order |
| "Leverage means I can afford more" | Leverage doesn't change your risk budget |
The takeaway#
Position sizing is the second half of the risk model begun in Part 3. Do the formula before every entry, keep per-trade risk to ~1%, and respect daily/weekly caps. With that in place, the variables you can still get wrong are strategy and execution — the parts backtestable from Part 4 and Part 5's session mechanics. Monkeys go broke by sizing up; professionals survive the drawdown by sizing down. That is the whole game that win-rate can't buy.
