In brief

  • Bitcoin miners have already created more than 18.5 million BTC.
  • This is over 88% of Bitcoin's total supply of 21 million.
  • Still, it will take another 120 years to find every last Bitcoin due to regular halvings.

The total number of mined Bitcoin (BTC) in circulation has reached over 18.5 million—out of a maximum of 21 million—over the past weekend, according to block explorer Blockchain.com.

There is now over 18.5 million BTC in circulation. Image: Blockchain.com

As a result, there is now less than 2.5 million BTC left for miners left to discover. However, while it might look like Bitcoin’s emission is closing on the finish line—just over 88% of all BTC are already mined, after all—the emission of the last Bitcoin is currently expected no earlier than the year 2140.

This is because, as time goes on, the rewards that miners receive for discovering a new Bitcoin block get smaller—slashed by half every 210,000 blocks (or roughly every four years)—due to a hardcoded process called the “halving.”

The last Bitcoin halving occurred on May 12 and reduced block rewards from 12.5 to 6.25 BTC per block. This will continue to happen every four years until the very last satoshi—Bitcoin’s smallest unit— is discovered.

Why only 21 million?

As Decrypt reported, it is not entirely clear why Bitcoin’s maximum emission was limited specifically to 21 million coins by the crypto’s creator Satoshi Nakamoto. However, there are some theories.

One explanation for the limit is the money supply replacement theory. An alternative suggestion is that the limit could be mathematically extrapolated from Bitcoin's operating parameters.

In the first instance, the entire world's money supply stood at approximately $21 trillion when Bitcoin was created. If it would become the world’s ultimate currency and replace all fiat, then each BTC would be worth $1 million while each satoshi would amount to $0.01. At the same time, while those two figures remarkably resemble each other, we can only guess whether it was a coincidence.

The second theory is a bit simpler. According to it, Bitcoin’s emission limit is mathematically tied to its halving cycles—since we roughly know when all the halvings are going to happen and can extrapolate it forward. As it stands, the sum of the block rewards for each cycle equals 100 (50 + 25 + 6.25 +3.125, etc). By multiplying this number by the 210,000 blocks/cycle figure, we get the maximum possible supply of 21 million.

And what will happen after all 21 million BTC are mined? Not much, really. The blockchain will continue to operate just as today—with the exception of miners’ rewards. Since no new coins would be discovered, miners will have to rely on transaction fees as the main source of income.

Luckily they have well over a century to prepare.