What it is

Volatility is the magnitude and speed of an asset's price fluctuations. A stock that swings two dollars on a calm day and eight dollars on a busy one is more volatile than a utility that barely moves. The challenge is turning that loose impression into a number you can actually trade with.

ATR - Average True Range does exactly that. It is a volatility gauge that reports a symbol's typical per-period move in price units. If a stock has an ATR of $1.50 on the daily chart, you should expect, on an ordinary day, roughly a dollar and a half of range from where price has been. ATR does not tell you direction. It only tells you size, how far price tends to travel in the period you are watching.

How it works

ATR is built from the true range, which is the largest of three distances on each bar:

1. The current high minus the current low.
2. The current high minus the previous close.
3. The current low minus the previous close.

The second and third measures capture overnight gaps that a simple high-minus-low would miss. ATR then takes a smoothed average of the true range over a lookback window, most commonly 14 periods. The result rises when bars get bigger and falls when they shrink, so ATR is always expressed in the same units as price, dollars for a US stock, points for an index.

Because it is an average, ATR lags. A single explosive bar nudges it up; it takes several quiet bars to bring it back down. That smoothing is a feature: it stops you reacting to one noisy candle.

How to use it

ATR earns its keep in two places: stops and sizing.

For stops, the idea is to place your exit beyond the noise. If a symbol routinely moves $1.50 a day, a stop only $0.30 away will be hit by random wiggle, not by your thesis being wrong. A common approach is to set the stop a multiple of ATR away from entry, say 1.5x or 2x ATR, so the distance scales with how active the market currently is. A trailing stop built on ATR widens in volatile conditions and tightens as things calm down.

For sizing, ATR tells you how many units to buy so that one ATR of adverse movement equals a fixed dollar risk. This is the bridge between volatility and position sizing: a volatile name gets a smaller position, a quiet name gets a larger one, and your risk per trade stays constant.

Worked example

Suppose your account risk per trade is $200, and a stock trades at $50 with a daily ATR of $2.

1. You decide your stop sits 2x ATR below entry, so $4 away ($46).
2. Risk per share is therefore $4.
3. Position size is $200 / $4 = 50 shares.

If ATR later rises to $3, the same 2x stop is $6 away, so the same $200 risk now buys only 33 shares. The market got noisier, so your position shrank automatically, exactly the behaviour you want.

Strengths & limits

ATR's strength is objectivity: it converts a vague sense of "choppy today" into a number you can size and place stops with, and it adapts as conditions change. It works on any timeframe and any liquid market.

Its limits matter too. ATR is directionless, it never says whether to be long or short. It is backward-looking, so it understates volatility right before a known catalyst like an earnings release. And the absolute value is symbol-specific: an ATR of $2 is huge for a $10 stock and trivial for a $1,000 one, so always read it relative to price or as a percentage.

Key takeaway: ATR measures how far a symbol typically moves per period, giving you an objective, adaptive yardstick for placing stops beyond the noise and sizing positions so risk stays constant.

Use ATR alongside a directional tool, never as a signal by itself.