Way of the Turtle — Chapter 2: Volatility Position Sizing (The N/ATR Unit Rule)

"The Turtle system's greatest invention was not the entry signal. It was using volatility to decide how much to buy." — Curtis Faith

Financial Context

Most retail investors size positions in dollars: "I'll put $100,000 into this stock." The Turtles sized in risk: "This trade may lose at most 1% of total capital."

The shift sounds small and is in fact the sharpest dividing line between amateur and professional execution. Sizing by dollars leaves your risk entirely determined by which instrument you happened to pick. Sizing by risk makes your exposure identical and known regardless of the instrument.

Wall Street Application

1. N Is the Average True Range

• Definition: N = the 20-day Average True Range, representing an instrument's normal daily movement.
• Why not a simple percentage of price: True Range accounts for overnight gaps, so it reflects real risk better than the intraday high-low spread alone.
• The core point: A stock that moves $5 a day and one that moves $0.50 a day must never be bought in the same share quantity — doing so takes ten times the risk in the first.

2. The Unit Formula

• Formula: Position size = (Account Equity × Risk %) ÷ (N × Value per point)

3. A Side-by-Side Calculation

With a $1,000,000 account risking 1% ($10,000) per trade:

• Instrument A (high-volatility tech): price $200, N = $8.00 Size = 10,000 ÷ 8 = 1,250 shares (about $250,000 of exposure)
• Instrument B (low-volatility utility): price $40, N = $0.80 Size = 10,000 ÷ 0.8 = 12,500 shares (about $500,000 of exposure)

Note that B's market value is twice A's, yet both carry identical risk. Under a "$300,000 in each" dollar-based approach, your risk in A would be far larger than in B — and you would have no visibility into that fact.

4. One Ruler Across Asset Classes

The same formula applies to equities, commodities, currencies, and ETFs because it measures volatility, not price. This is precisely what allowed the Turtles to trade a dozen unrelated markets simultaneously.

Risk Management Rules

1. Always compute N before share count: Check the 20-day ATR before every order; never size by dollar intuition. Most broker platforms include ATR as a standard indicator.
2. Cap single-trade risk at 1%: For retirement accounts, tighten below 0.5%. The smaller the account and the nearer the withdrawal need, the more conservative this should be.
3. Cap aggregate exposure: Limit highly correlated positions (several names in one sector) to four total units, so no single macro factor can breach the portfolio at once.
4. Recompute N regularly: Volatility shifts with market regime. Recalculate at least monthly while holding, and immediately during turbulent markets.

Relevance to a Retirement Portfolio

Even if you never trade actively, this way of thinking is useful: it reveals that equal-dollar diversification is not equal-risk diversification. In a portfolio of high-volatility growth stocks and low-volatility bonds, essentially all the real risk comes from the former. Examining your retirement portfolio through volatility rather than dollars usually reveals far more concentrated risk than expected.