The standard statistical measure of volatility is the standard deviation of daily returns. ATR is different: it measures the typical daily range (high minus low, adjusted for overnight gaps). The two measures are correlated but not identical. ATR captures the intraday excursion - how much the price typically moves during a single day, including the overnight gap. A stock with a steady daily uptrend (close-to-close return 1% per day, intraday range 0.5%) has low ATR; a stock with a flat close-to-close trend but wild intraday swings has high ATR.
Figure 1. Concept map for this episode: fight, desk translation, what to lock.
The definition
For a single day, the True Range is the maximum of: - High − Low (today's range) - |High − PriorClose| - |Low − PriorClose|
ATR-based stops
The standard ATR-based stop is placed some multiple of ATR below the entry price - typically 1.5 to 2.5 times ATR. The target is placed symmetrically on the other side, scaled by your target payoff ratio (for example, 2x the stop distance for a 2:1 target).
The 2% rule, translated
The classical "risk 2% of capital per trade" rule (Chapter 6) becomes, with ATR-based stops: divide your 2% risk budget by the stop distance (the ATR multiple times the ATR) to get your share count. For ₹10 lakh of capital, a 1.5x ATR multiple, and a stock with ATR = ₹3: that's ₹20,000 / (1.5 × 3) ≈ 4,444 shares. If the same stock's ATR doubles to ₹6 during a vol spike, the share count works out to ₹20,000 / (1.5 × 6) ≈ 2,222 shares - the position size halves automatically, and the dollar risk stays fixed at ₹20,000 either way.
Definition
The True Range incorporates overnight gaps by comparing today's high and low to the previous close, not just to today's open. This matters because a stock can gap up 5% overnight and then trade in a tight range - the high-minus-low would understate the volatility a stop must survive, but the high-minus-prior-close captures it correctly. The True Range is always non-negative (it is the maximum of three non-negative quantities) and is a more robust intraday-volatility estimator than the simple high-low range.
Worked picture
Suppose a stock has been quiet for two months: ATR = ₹2. The system has been buying 5,000 shares per trade, risking ₹15,000. A surprise earnings announcement triggers a vol spike. Over a week, ATR rises to ₹4. The system automatically halves the position size to 2,500 shares, still risking ₹15,000. The trader does not have to decide "should I cut my size?" The ATR-based sizing rule has made the decision for them. The realised risk per trade stays constant; the share count adjusts to match the new volatility regime. This is the operational discipline at its best: a simple rule, applied mechanically, produces the behaviour that a sophisticated risk manager would prescribe - without requiring discretionary judgement in the heat of the moment.
What breaks in production
If a stock has just transitioned from a calm to a volatile regime, the 14-day ATR is a lagged indicator - it still reflects the old regime mixed with the new. The sizing computed from such an ATR will be too aggressive for the new reality. The fix is hybrid: use a fast (e.g., 5-day) ATR as a floor and the standard 14-day as a baseline. Take the maximum of the two. This way, recent volatility spikes are reflected immediately, while sustained calm regimes are not under-sized. A more elaborate fix uses intraday vol (computed from minute bars) to detect regime transitions in real time and update the sizing ATR within the day. For most strategies, the simpler hybrid is sufficient.
Key idea
ATR-adaptive sizing is the practical instantiation of "size in inverse proportion to volatility." It is a one-line formula that captures one of the most important risk-management principles in trading. Combined with the friction governor (Chapter 25), the pre-flight circuit breaker (Chapter 26), and the concentration limits (Chapter 31), it forms the defensive backbone that protects an edge from the inevitable volatility shocks of real markets.