What is the Bitcoin halving, and how does it change the reward?

The rule

Bitcoin's code starts the block subsidy at 50 bitcoin and cuts it in half every 210,000 blocks.² At about 10 minutes a block, 210,000 blocks take roughly four years.¹ This cut is called the halving.¹

A halving only changes the subsidy. Miners still keep the fees, so fees become a bigger share of their income after each halving.¹

Every halving so far

The date of a future halving can only be estimated, because block times vary. Halvings often land a little earlier than a simple calendar guess, as blocks come slightly faster when mining power grows.¹

Working out the subsidy

The code counts in satoshis. It divides the block number by 210,000 and drops any remainder. That gives the number of halvings so far. Then it halves 5,000,000,000 sats (50 BTC) that many times.^2,4

Where 21 million comes from

Each 210,000-block period creates 210,000 × the subsidy.² The first period made 10,500,000 coins, half of the total.⁵ The next periods made 5,250,000, then 2,625,000, then 1,312,500.⁵ Each period adds half as much as the one before, so the total creeps towards 21 million.^5,1

Because the code counts in whole satoshis, the subsidy is rounded down to zero after the 33rd halving.⁴ New bitcoin stops around the year 2140.^4,1 After that, miners will be paid by transaction fees only.⁴

The reward in one block

A miner's full reward is the subsidy plus the fees of every transaction in its block.⁶ The reward can't be spent until at least 100 more blocks have been added.⁶