Quick TakeExchanges hold roughly 3.08 million BTC on behalf of customers, which is 16.5% of all available bitcoinsThe amount is down by 10% since February 9 when exchanges held as much as 3.43 million BTC (nearly 19% of the available supply)Coinbase holds by far the most of all exchanges – 984.3k BTC, which is 5.4% of all BTC in circulation