Implied volatility is derived from option prices: what the market charges today for movement between now and expiry. Realised volatility is computed from price history: how much the asset actually moved.
The first is a forward price, the second a backward measurement, and the relationship between them is where option sellers make or lose money.
The structural gap
Across most markets and most periods, implied sits above subsequent realised. That premium is compensation for taking the other side of uncertainty, and it is why systematically selling options makes money most of the time and loses badly in the tail.
Crypto is no exception to the premium. What differs is its size and its stability — both larger and less reliable than in equity index options, because the underlying regime shifts more.
Measuring realised without lying to yourself
Realised volatility depends on the window and the sampling, and both are choices that change the answer. A short window over a violent stretch produces a number that will not persist; a long one smooths away the condition you were trying to measure.
Crypto adds a wrinkle that equities do not have: weekends trade. So there are no non-trading days to exclude, and the annualisation factor differs from the equity convention. Applying an equity trading-day count to a market that trades every day produces a number wrong by a fixed multiple, permanently and invisibly.
What the comparison is good for
Deciding whether options are expensive, which is a different question from whether the market will go up or down. Implied far above trailing realised means the market is paying up for movement it has not seen — sometimes correctly, ahead of a known event.
And read beside DVOL and the term structure, it locates when the market expects that movement, which the single implied number does not.