Altcoin Season Index

The index compares each of the 50 largest altcoins by market capitalization with bitcoin over the same 90-day window and reports the share that did better. Stablecoins and wrapped or tokenized versions of other assets are usually left out, because their prices follow something else. A reading of 75 means three in four large altcoins outperformed bitcoin; 25 or less is called bitcoin season, and the range in between is neutral.

The idea was popularized by the Blockchaincenter website and is now published by several data sites, with small differences in the coin list and the time window. Traders use it to judge whether money is rotating from bitcoin into riskier tokens, something that has tended to happen late in bull markets, as in early 2018 and during 2021, and they often read it alongside bitcoin dominance.

It is a backward-looking measure of breadth, not a signal of what comes next. A high reading can arrive just as a rally ends, a handful of large tokens can rise together for reasons unrelated to a broad rotation, and the top 50 changes as new tokens enter and others drop out, so readings from different years are not strictly comparable.