The single most important skill in trading — determining how many shares to buy based on risk, not desire. Fixed-percentage, ATR-based, and Kelly Criterion methods.
1. Explain why position sizing — not stock picking — is the primary determinant of trading survival.
2. Apply the fixed-percentage risk model (risking 1–2% of account per trade).
3. Calculate position size using the ATR-based method.
4. Understand the Kelly Criterion and why it should be used conservatively.
5. Scale position size based on conviction level and market conditions.
6. Avoid the common sizing mistakes that blow up trading accounts.
If you remember nothing else from TradeCraft, remember this: position sizing determines whether you survive as a trader. You can have a mediocre strategy with excellent position sizing and make money. You can have a brilliant strategy with terrible position sizing and go broke. The math is unforgiving.
Consider two traders with identical win rates (55%) and identical average win/loss sizes. Trader A risks 2% of their account per trade. Trader B risks 20% per trade. Both hit a losing streak of 5 trades in a row — which happens regularly even with a 55% win rate. Trader A's account drops from $100,000 to $90,392 (a 9.6% drawdown — painful but survivable). Trader B's account drops from $100,000 to $32,768 (a 67% drawdown — catastrophic). To recover from 67% down, Trader B needs a 200% gain. Trader A needs an 11% gain.
Position sizing is risk management. Everything else — stop-losses, diversification, hedging — is secondary to the fundamental question of how much capital you expose to any single trade.
The fixed-percentage model is the most widely used and recommended approach for non-professional traders. The rule: never risk more than 1–2% of your total account on any single trade.
The calculation:
Risk per trade = Account size × Risk percentage
Position size (shares) = Risk per trade ÷ Distance to stop-loss
Example: You have a $50,000 account and risk 1% per trade ($500). You want to buy a stock at $85 with a stop-loss at $80 (risk = $5/share). Position size = $500 ÷ $5 = 100 shares ($8,500 position).
Notice the position size is derived from your risk, not from how much cash you have or how much you "like" the stock. This is the core principle: risk defines size, not desire.
At 1% risk per trade, even a devastating losing streak of 10 consecutive losses (which happens more often than you'd think) only reduces your account by about 9.6%. You're still fully functional. At 5% risk, that same streak takes 40% of your account. At 10%, you lose 65%. The 1–2% rule ensures that no single trade or short losing streak can destroy your ability to continue trading.
| Risk Per Trade | After 5 Losses | After 10 Losses | Gain Needed to Recover |
|---|---|---|---|
| 1% | −4.9% | −9.6% | 10.6% |
| 2% | −9.6% | −18.3% | 22.4% |
| 5% | −22.6% | −40.1% | 66.9% |
| 10% | −41.0% | −65.1% | 186.8% |
| 20% | −67.2% | −89.3% | 834.6% |
Losses and recoveries are not symmetric. A 50% loss requires a 100% gain to recover. A 75% loss requires a 300% gain. This is why preventing large drawdowns is more important than chasing large gains. The fixed-percentage model keeps drawdowns manageable, preserving your capital for the opportunities that inevitably follow losing streaks.
The ATR method (introduced in Module 5.6) uses the stock's own volatility to set the stop distance, then calculates position size from there. This automatically adjusts your position size for volatile vs. calm stocks.
The formula:
Stop distance = N × ATR (typically N = 1.5 to 2)
Position size = Risk per trade ÷ Stop distance
Example: Stock at $120 with a 14-day ATR of $4. Using a 2× ATR stop: stop distance = $8. With $500 risk: position size = $500 ÷ $8 = 62 shares ($7,440).
Compare this to a low-volatility stock at $120 with ATR of $1.50: stop distance = $3, position size = $500 ÷ $3 = 166 shares ($19,920). The ATR method automatically gives you a larger position in the calmer stock and a smaller position in the volatile one — matching your risk exposure to the stock's actual behavior.
Open a spreadsheet and create a position-sizing calculator with these inputs: Account size, Risk % (1%), Entry price, Stop price. The formulas: Risk $ = Account × Risk %. Stop distance = Entry − Stop. Shares = Risk $ ÷ Stop distance. Position value = Shares × Entry price. % of account = Position value ÷ Account. Run three scenarios: a tight stop (2% from entry), medium stop (5%), and wide stop (8%). Notice how the position size shrinks as the stop widens — this is the model working correctly, automatically reducing your exposure when uncertainty is higher.
The Kelly Criterion is a formula that calculates the mathematically optimal fraction of capital to risk: Kelly % = W − (1 − W) / R, where W = win probability and R = reward/risk ratio.
Example: Win rate = 55% (W = 0.55), average win = $600, average loss = $400 (R = 1.5). Kelly % = 0.55 − (0.45 / 1.5) = 0.55 − 0.30 = 25%.
The problem: Full Kelly is extremely aggressive and produces massive drawdowns that most humans cannot endure psychologically. A 25% risk per trade sounds mathematically optimal but will produce 50%+ drawdowns regularly. In practice, professional traders use half-Kelly or quarter-Kelly — cutting the recommended size to 6–12% or 3–6%. This sacrifices a small amount of theoretical long-term growth in exchange for dramatically reduced drawdowns and much smoother equity curves.
For most non-professional traders, the fixed 1–2% rule is simpler and more robust than Kelly, because Kelly requires accurate estimates of win rate and payoff ratio — which are difficult to know with precision, especially early in your trading career.
Not all trades deserve the same position size. A framework for scaling:
| Conviction Level | Risk per Trade | When to Use |
|---|---|---|
| High conviction (A+) | 2% of account | All timeframes aligned, strong fundamental thesis, multiple technical confirmations, favorable macro |
| Standard | 1% of account | Good setup with one or two minor caveats (counter-trend element, mixed volume) |
| Speculative | 0.5% of account | Lower-probability setup, binary event, unfamiliar stock, testing a new strategy |
Individual trade sizing is necessary but not sufficient. You also need portfolio heat limits — the total risk across all open positions. If you have 10 open positions each risking 2%, your total portfolio heat is 20%. A market shock that hits all positions simultaneously could cause a 20% drawdown. A common rule: total portfolio heat should not exceed 6–8%. This means at 1% risk per trade, you can have 6–8 positions. If you want more positions, reduce per-trade risk.
Module 7.2 covers stop-loss strategies that determine the "distance to stop" input for position sizing. Module 7.3 introduces risk/reward ratios that determine which trades are worth taking at all. Module 4.6 (Thesis) defines the conviction framework used for scaling. The three modules together create a complete risk management system: thesis defines conviction → position sizing determines how much → stop-loss determines where → risk/reward determines whether.
In March 2020, the S&P 500 dropped 34% in 23 trading days. Two traders entered the year with $100,000 accounts and similar strategies (long individual stocks with technical entries).
Trader A used the 1% rule religiously. With 6 open positions, each risking 1%, their maximum exposure was 6%. When the crash hit, several stops were gapped through (gap risk — Module 6.4), and actual losses were closer to 1.5% per position. Total drawdown: approximately 12%. Painful, but the account at $88,000 was fully functional. Trader A had the capital — and the psychological stability — to start buying the March 23 bottom.
Trader B sized positions based on "conviction" without a formal model — holding 4 large positions at roughly 25% each with wide mental stops (no actual stop orders). When the crash hit, all four positions fell 30–40%. The account dropped from $100,000 to roughly $35,000 — a 65% drawdown. Trader B panic-sold at the bottom, locked in the losses, and missed the subsequent recovery. The account never recovered.
Same market, same timeframe, similar strategies. The difference was position sizing. Trader A's 1% rule transformed a historic crash into a survivable drawdown. Trader B's lack of sizing discipline transformed it into a career-ending event.
1. You have a $40,000 account, risk 1% per trade, and want to buy a stock at $60 with a stop at $57. How many shares should you buy?
2. Why is the 1–2% rule more important than stock selection for trading survival?
3. A stock has a 14-day ATR of $5. Using a 2× ATR stop, your stop distance is $10. With $500 risk budget, what is your position size?
4. The Kelly Criterion suggests risking 25% per trade. Why do professionals typically use quarter-Kelly (6.25%) instead?
5. You have 8 open positions, each risking 1.5% of your account. Your portfolio heat is:
6. A high-conviction A+ trade should receive what percentage of account risk?