What To Do When Volatility Spikes

Position sizes are usually set relative to how much a stock or asset typically moves — a trader sizes a position so that a normal day's move costs an acceptable, known amount. When volatility spikes, that relationship breaks silently: the position hasn't changed size, but the range of outcomes it can produce on a given day has widened sharply, which means the risk actually being carried has grown even though nothing was traded.

Why the size that was right yesterday is wrong today

A trader sizes a $500k position in a stock that typically moves 1.5% a day, meaning a bad day costs roughly $7,500 — an acceptable number against the trader's risk budget. Then the stock's volatility jumps to 4% a day, following, say, a surprise regulatory investigation. The position is unchanged at $500k, but a bad day on it now costs closer to $20,000 — nearly three times the number the size was originally chosen to produce. Nobody made a decision to take on more risk. The market simply changed the meaning of the same position size.

The instinct to fight, and why it's wrong

The natural instinct when vol spikes on a position you already like is to hold the size, on the reasoning that the thesis hasn't changed. But the thesis and the appropriate size are two separate questions — a stock can still be a good long-term holding while being, for the next two weeks, roughly three times riskier per dollar than it was before, which means holding the same dollar amount is quietly a decision to take on three times the intended risk, whether or not anyone meant to make that decision.

A scenario

A trader runs five positions, each sized to a similar daily risk budget of roughly $8,000 based on trailing volatility. A market-wide shock — an unexpected central bank move — causes realized volatility across the board to roughly double overnight.

Position	Size before	Daily vol before	Daily risk before	Daily vol after shock	Daily risk if size unchanged
A	$530k	1.5%	~$8,000	3.1%	~$16,400
B	$400k	2.0%	~$8,000	3.8%	~$15,200
C	$800k	1.0%	~$8,000	2.4%	~$19,200
D	$267k	3.0%	~$8,000	5.5%	~$14,700
E	$1,000k	0.8%	~$8,000	2.2%	~$22,000

If the trader does nothing, the book's total daily risk roughly doubles, from about $40,000 to about $87,500, without a single new trade. The trader who checks and cuts each position back toward its original $8,000 daily-risk target — roughly halving each size — is back to running the same intended risk as before the shock, on the same theses, just at sizes that match the new volatility rather than the old.

The trap in the other direction

The mirror-image mistake is cutting size on everything the moment vol ticks up even slightly, which throws away genuinely good positions on the assumption that any volatility increase is dangerous. Not every vol increase warrants action — a modest, temporary bump around a routine data release is different from a sustained regime shift. The useful trigger is a vol increase large enough and persistent enough to meaningfully change the dollar risk of the position, not any wiggle in the number.

What happens to trading costs at the same time

Spiking volatility usually arrives together with wider bid-ask spreads and less size available at each price level, which means resizing a position costs more to execute than it would have the day before the spike. That's a real cost, but it's not a reason to skip resizing — it's a reason to resize promptly, before spreads widen further, rather than waiting for the situation to feel more settled.