IV rank and IV percentile
Lesson 14 · about 8 min
A 32% IV is high for an index and low for a small-cap biotech. To decide whether options on a given stock are expensive right now, you need to compare today's IV to that stock's own history. Two measures do that job, and they are frequently confused.
IV rank
IV rank places today's IV on a scale between the lowest and highest IV of the past year:
IV rank = (current IV − 52-week low IV) ÷ (52-week high IV − 52-week low IV) × 100
XYZ's IV today is 32%. Over the past year its IV ranged from 20% to 60%.
IV rank = (32 − 20) ÷ (60 − 20) × 100 = 12 ÷ 40 × 100 = 30
A rank of 30 says today's IV is 30% of the way from the year's low to the year's high. Rank 0 is the annual low; rank 100 is the annual high.
IV percentile
IV percentile is the share of trading days in the past year on which IV was below today's level.
Suppose that of the last 252 trading days, XYZ's IV was under 32% on 176 of them.
IV percentile = 176 ÷ 252 × 100 ≈ 70
A percentile of 70 says IV has been lower than today on 70% of days this year.
Why they disagree
XYZ has rank 30 and percentile 70 at the same moment. How? Because the year's range of 20% to 60% was set mostly by one spike: IV was 60% for a few days around a bad earnings report and spent most of the year between 22% and 30%. Rank is dragged around by the extremes; percentile looks at the whole distribution.
| Measure | What it compares against | Distorted by | Better for |
|---|---|---|---|
| IV rank | The year's high and low | A single spike or crash | Quick read when the year was calm |
| IV percentile | Every day in the year | Nothing in particular; slower to react | Deciding whether today is unusual |
Most experienced traders prefer percentile, or use both and note when they diverge, because a divergence itself is information: it means the year contained an outlier.
A second worked example
A different stock, ABC. IV today 45%. 52-week range 40% to 95%. Days below 45% in the past year: 60 of 252.
- IV rank = (45 − 40) ÷ (95 − 40) × 100 = 5 ÷ 55 × 100 ≈ 9.
- IV percentile = 60 ÷ 252 × 100 ≈ 24.
Both say IV is low for ABC. Yet 45% is objectively a lot of expected movement (about 13% over 30 days). Low relative to its own history does not mean cheap in absolute terms; it means ABC is usually even wilder than this. The correct reading is "options on ABC are near the cheap end of their usual range", not "options on ABC are cheap".
Key idea: IV rank measures where today sits between the year's extremes; IV percentile measures how often the year has been lower than today. Percentile is harder to distort. Both say "high or low for this stock", never "high or low in general".
How traders use them
The common heuristic is:
- High rank/percentile (say, above 50): options are expensive relative to the stock's own history. Strategies that sell premium (credit spreads, covered calls, cash-secured puts) collect more, and a reversion toward normal IV helps them. Buying outright options here means paying up and facing vega losses if IV normalises.
- Low rank/percentile (below 25 or so): options are cheap relative to history. Buying options or debit spreads costs less and vega works for you if IV rises. Selling premium here collects little and has less cushion.
Two cautions. First, high IV is usually high for a reason: an event is coming, or the stock has been moving violently. "Sell high IV" is not a rule; it is a prompt to find out why IV is high and decide whether the market is overpaying for that reason. Second, low IV can stay low for months; buying options because "IV is cheap" without a view on the stock is a slow bleed through theta.
The one number to write down
For any option trade, note the IV percentile at entry in your log next to the price. Over a hundred trades you will be able to see whether your buys work better from low percentiles and your sells from high ones. That is a real edge test, and it is impossible without the number.
Try it: Find IV rank and IV percentile on your platform for five stocks. For any where the two differ by more than 20 points, look at the one-year IV chart and find the spike or crash that explains the gap. Decide which measure you would trust for that stock today.
Recap
- IV rank = (current − 52-week low) ÷ (52-week high − 52-week low) × 100; it compares today to the year's extremes.
- IV percentile = share of days in the past year with IV below today's; it compares today to the whole distribution.
- Rank is distorted by a single outlier; percentile is not, so prefer it or use both.
- High readings favour selling premium and warn buyers about vega; low readings favour buying and warn sellers about thin premium.
- Both measures are relative to the stock's own history; a "low" IV on a volatile stock can still be a big expected move.
See it drawn
Original diagrams for the ideas on this page. Illustrative, not real market data.