Bitcoin's 21 million cap is actually a ceiling it will never quite reach

The 21 million BTC supply limit is one of the most cited numbers in crypto, but the protocol will never actually issue the full amount. Block rewards are denominated in whole satoshis — the smallest unit of bitcoin, at 0.00000001 BTC — and the halving schedule repeatedly cuts those rewards in half. Once the reward drops below one satoshi, it rounds down to zero, meaning the final fractions of that theoretical ceiling can never be minted.

The actual maximum supply is estimated at roughly 20,999,999.9769 BTC. The gap is tiny and economically trivial, but it reflects something precise about how Satoshi Nakamoto designed the issuance schedule: integer arithmetic at the satoshi level takes precedence over the round number, and the round number was always an approximation.