STD DEV CALCULATOR
How far is "normal" for this index right now? Paste closing prices, get the range a move actually has to clear before it means something.
What This Actually Tells You
Standard deviation measures how much daily returns typically bounce around their average. A low std dev means the index has been calm — small, predictable daily moves. A high std dev means it's been volatile — big swings are normal right now, not exceptional.
The number by itself doesn't tell you direction. What it gives you is a yardstick: once you know the typical daily move, you can tell whether today's move is ordinary or a genuine outlier — which is exactly the judgment call behind "is this a real breakout or a fakeout that reverts."
WHEN TO USE IT
Before sizing a stop, before trusting a breakout, or before deciding if today's range is "trend day" wide or ordinary. Recalculate it as volatility regimes shift — a std dev from a calm month is stale once the market gets choppy.
1 / 2 / 3 SD, IN PLAIN TERMS
A 1-SD move happens often — roughly two days out of three. A 2-SD move is uncommon, maybe once every few weeks. A 3-SD move is rare enough that it usually means something changed (news, a real regime shift), not just noise.
FOR STOP SIZING
A stop tighter than 1 SD gets clipped by ordinary noise, not just wrong reads. Sizing stops relative to std dev — not a fixed point count — adapts automatically as volatility changes.
THE CAVEAT
This assumes returns are roughly normal-distributed, which markets only approximate — real moves have "fatter tails" than a textbook bell curve. Treat 3-SD odds as a rough guide, not a hard probability.
Calculate It
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AVG DAILY MOVE
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STD DEV (DAILY)
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ANNUALIZED VOL