Volatility
How much and how quickly an asset’s price moves over a given period.
Also called: vol · volatile
Written by Javier Sánchez Ros
In plain language
Volatility measures the size of price swings, not their direction. A market that falls 3% and rallies 3% every day is highly volatile whether or not it ends the week higher.
It is usually quantified as the standard deviation of returns, or in trading terms via Average True Range, which reports the typical daily range in the instrument’s own price units.
Volatility clusters. Quiet periods tend to follow quiet periods, and once a market becomes violent it usually stays that way for a while.
Worked through
A 2% stop on a utility and on a small-cap biotech
- Utility: typical daily range
- ~0.8%
- A 2% stop is
- 2.5 average days
- Biotech: typical daily range
- ~6%
- A 2% stop is
- a third of one day
The same rule, applied honestly, produces a sensible trade in one case and a guaranteed loser in the other. On the utility, 2% is genuine room; on the biotech it is inside the ordinary noise of a single session, so the stop is taken out by movement that carries no information at all.
The trader will read this as the setup failing. It is not — the setup was never given a chance to be right or wrong, because the exit was placed inside the instrument’s normal breathing.
Volatility is what makes a fixed percentage stop meaningless across instruments. The distance has to be measured in units of how much this thing moves, which is what average true range is for, and only then converted into a percentage.
Position size then does the rest. A wider stop on the biotech is not more risk — it is a smaller position for the same risk, which is the correct response to a more violent instrument rather than a reason to avoid it.
Why it matters
Volatility should set your stop distance, and your stop distance sets your position size. Using the same stop on a calm and a violent instrument means taking wildly different real risks.
Common mistakes
- Using a fixed percentage stop across instruments with completely different ranges.
- Confusing volatility with opportunity. More movement also means more ways to be stopped out.
- Sizing up during quiet periods and forgetting that volatility can triple overnight.
Keep exploring
These concepts are connected. Understanding one usually makes the next one easier.
The average size of an instrument’s price range per period, including gaps.
A stop placed at a multiple of the instrument’s Average True Range rather than a fixed percentage.
The gap between your entry and your stop loss — your risk on a single unit.
The market’s expectation of future price movement, derived from option prices.
The amount of an asset you buy or sell in a single trade.