Altcoin season index rose while 73 of 95 coins fell

Measured at 14:43 UTC on CoinGecko data, 36 of the 94 largest coins outside stablecoins beat bitcoin over 30 days, and 42 beat it over seven. The share that beats bitcoin went up, and over those same seven days only 22 of 95 coins were worth more at the end than the start.

“A share that rises because the benchmark fell is not the same measurement as a share that rises because the constituents rose, and an index cannot tell you which one you are looking at.”

— Intokened, Editorial note, 15 September 2026

Our reading of what the breadth numbers do and do not say

What the two frames show

Both facts are true and they point in opposite directions. Read the seven-day number alone and the market looks like it is rotating into altcoins. Read the list of prices and almost three quarters of it is red. Alts are not beating bitcoin by rising. They are beating it by falling less.

Three pairs of numbers hold the whole picture:

• Over 30 days bitcoin is up 21.40% and the median altcoin is up 15.80%, a gap of 5.6 points.
• Over 7 days bitcoin is down 2.50% and the median altcoin is down 2.60%, a gap of one tenth of a point.
• Over 24 hours bitcoin is down 3.38% and the median altcoin is down 1.88%.

The thirty-day frame is the one that describes a rally. Bitcoin leads it, and 77 of the 95 coins we measured are still above where they stood a month ago. That is a broad advance with a clear leader, which is close to the opposite of what people mean when they say altcoin season.

The seven-day frame describes the last few sessions of it coming off. Total market value fell 4.52% in a day while we were counting, bitcoin traded at $75,765, and dominance sat at 58.2% with ether at 11.3%. The gap between bitcoin and the median altcoin over that week is one tenth of a percentage point, which is another way of saying nothing rotated anywhere.

Altcoin season index: why the share rose

Our own altcoin season index counts the share of the top 100 beating bitcoin over 30 days, and that number is doing what it is built to do. The error is in the reading rather than the arithmetic. A share is a ratio, and a ratio moves when either half moves. When the denominator is a falling bitcoin, the ratio improves on a day nobody enjoyed.

The spread inside the list is wider than the averages suggest. The strongest coin in the top 100 is up 180.3% over 30 days, zcash is up 133.8%, and Uniswap and Arbitrum are up 98.3% and 90.0%. At the other end the worst performer is down 7.2%. A median of plus 15.8% sits in the middle of that range and describes almost none of it, which is why a single market number should be a starting question rather than an answer.

Into the rate decision

All of it faces a rate decision tomorrow at 18:00 UTC, where traders price a better than 87% chance of the first rise since 2023. We measured yesterday that bitcoin now moves less on Fed days than it did a year ago, even though it swings harder the rest of the time. Breadth this thin going into that is worth knowing, and it is not the same claim as knowing what happens next.

Informational material, not investment advice. All figures were measured at 14:43 UTC on 15 September 2026 and move with the market.