What's being tracked
Exchange reserves sums the bitcoin balance sitting in known exchange-labeled wallets at a point in time, expressed as a percentage of total circulating supply. Unlike netflow (a daily rate), reserves is a running stock — effectively netflow accumulated since exchanges began operating, adjusted for supply growth from mining.
The multi-year downtrend
Since Coin Metrics' tracked data begins, exchange reserves has trended down from a historical high around 17% of circulating supply to well under half that level today. The dominant explanation offered for this trend: the rise of self-custody culture, cold-storage services, ETF share structures that hold underlying bitcoin off exchange, and a general post-2014 (Mt. Gox) wariness about leaving coins on trading platforms. None of that is directly measurable from the reserve number itself — it's the most common interpretation layered on top of a real, measured trend.
Where the "bullish" framing gets oversold
A falling reserve percentage means less of the circulating supply is immediately available to sell on an exchange without first being deposited. It does not measure demand, and it does not distinguish "long-term holder moving to cold storage" from "exchange itself losing market share to a competitor" or "coins moving to a custodian that isn't labeled as an exchange in the dataset." Reserves have never fallen below roughly 9% of supply even at the low end of their historical range — a real floor worth knowing about, well above zero.
Today's exact reserve percentage, calibrated against the real historical 8%–18% range, updates live on the Exchange Reserves chart.