Altcoin Season Index

What does the Altcoin Season Index measure?

The Altcoin Season Index is a number from 0 to 100 that tries to answer one specific question: what share of the largest coins in the market has beaten Bitcoin's return over a given period? If most of the largest coins are outperforming Bitcoin, the index sits high. If, conversely, Bitcoin is outperforming most other large coins, the index sits low.

The index is published by CoinMarketCap, and it's built on their own thresholds for when the market is characterized as one or the other: above 75 is called "altcoin season," below 25 is called "Bitcoin season," and the area in between is considered a neutral or mixed market. It's worth noting that CMC doesn't publish the precise methodology in detail — including exactly which coins are included and over what length of period the returns are compared — so we deliberately avoid stating exact figures for the number of coins or the lookback window here. What can be said with confidence is the principle: a comparison of the largest coins' returns against Bitcoin's over a period, distilled into a single number.

Because the index is built on "the largest coins" rather than a fixed, named list, the composition of which coins are included isn't static — it shifts as new coins grow into the top of the market and others fall out.

How do you read it?

The index is typically used as a temperature gauge for the market's risk appetite. When the index is high (above 75, altcoin season), the interpretation is that capital and interest have broadly shifted away from Bitcoin and out into a wider field of altcoins — a sign historically associated with more speculative, risk-seeking phases of the market cycle. When the index is low (below 25, Bitcoin season), the interpretation is the opposite: the market is more defensive, and capital is consolidating around Bitcoin as the most established asset.

Viewed over time on the chart above, the index typically moves in broad waves rather than fast swings — it doesn't flip from Bitcoin season to altcoin season overnight, but shifts gradually as more and more of the largest coins start or stop beating Bitcoin. That makes the index well suited to describing which phase the market is in right now, rather than predicting when the phase will change.

The index is naturally linked to Bitcoin dominance (see the separate page): when altcoins as a group beat Bitcoin, dominance will typically fall at the same time, and vice versa. The two figures measure related but not identical things — dominance is about market cap, the altcoin season index is about returns — and they can therefore move with a degree of lag relative to each other.

When does the indicator lie?

The most fundamental weakness of the index is that it compares returns over a period looking backward — it's a slow measure by construction. Because it looks at how the largest coins have performed against Bitcoin over a period, the index doesn't react to sharp, fast shifts in market sentiment. A market can start rotating hard into altcoins without the index moving much at all, because the newer move doesn't yet carry enough weight in the overall picture across the whole period.

Another significant weakness is that "the largest coins" isn't the same group from year to year. The composition of which coins count as "large enough" to be included shifts continuously, as new projects grow into the top of the market and older coins fall out or lose relevance. That means the index doesn't, in practice, compare Bitcoin against the same basket of altcoins over time — it compares Bitcoin against whatever happened to be among the largest coins at that particular moment. Two readings taken years apart can therefore describe two genuinely different groups of coins, even though the number looks like it's measuring the same thing.

Finally, like Bitcoin dominance, the index is an aggregate measure for the whole market and doesn't say anything about which specific sectors or narratives are driving the move. A high index reading can just as easily stem from broad strength across many different coins as from a single sector pulling the average up sharply while the rest of the market is actually lagging Bitcoin.

How we use it

We follow the Altcoin Season Index as a complement to Bitcoin dominance and the Fear & Greed Index — none of the three gives a complete picture alone, but together they form a more robust picture of which phase of the cycle the market is in. We've written more about how rotations between Bitcoin and altcoins have historically played out, and what has characterized the transitions, in our articles.