The Altcoin Season Index measures the percentage of the top 50 altcoins (excluding stablecoins) that have a higher N-day ROI than the season standard (default: Bitcoin). When fewer than 25% of altcoins outperform BTC, it's Bitcoin Season — BTC is outperforming most altcoins. When more than 75% of altcoins outperform BTC, it's Altcoin Season — altcoins are broadly rallying. The shaded green zone (0–25) marks Bitcoin Season territory, and the red zone (75–100) marks Altcoin Season. Data source: CryptoCompare daily OHLCV. The top 50 coin list is curated and excludes stablecoins and wrapped/asset-backed tokens. For the historical chart, the number of available altcoins decreases the further back you go, so the index is calculated from whatever coins have sufficient price history at each point in time.