How does the halving change Bitcoin’s subsidy?

How does the mechanism work?

Bitcoin’s main-network subsidy began at 50 BTC per block and halves every 210,000 blocks. The rule produces the familiar supply cap of 21 million BTC. Integer rounding in satoshis matters when describing exact eventual issuance.

The block reward can include both the permitted subsidy and transaction fees. A miner cannot make a block valid simply by claiming more new coins. Nodes validate the issuance transaction’s amount against the subsidy and fees allowed for that block.

Keep three quantities apart when reading a block: scheduled subsidy, permitted reward and actually claimed reward. The schedule supplies the issuance ceiling for that height. Adding transaction fees gives the maximum issuance transaction output sum. The miner’s transaction states what it claims within that allowance. A chart based only on the schedule describes an upper-bound model, while an account of actual issuance needs historical block data and relevant exceptions. This calculator intentionally performs the schedule calculation alone. That narrow question can be answered exactly from height without presenting a simplified supply curve as a measurement of coins available to spend.

Which subsidy applies at an exact boundary?

Calculate the subsidy era by integer division of block height by 210,000. At height 839,999 the quotient is 3; at height 840,000 it is 4. Begin with 5,000,000,000 satoshis, the 50 BTC initial subsidy, and shift right by the era count. The allowed subsidy falls from 625,000,000 to 312,500,000 satoshis at that boundary.

In BTC those amounts are 6.25 and 3.125 respectively, using 100,000,000 satoshis per BTC. The calculation takes a height, not a date. You can enter both boundary heights in the offline calculator and compare the results without a live block feed. The calculator reports a rule for a hypothetical block at your chosen height, not the height the network has reached.

For an invented reward example, suppose the height-840,000 block contains transactions paying 20,000 satoshis in fees. Its permitted issuance transaction output sum is at most 312,520,000 satoshis: scheduled subsidy plus those fees. Fees are transferred value; subsidy is newly issued value. A miner may claim less, but cannot claim more and have the block pass normal validation.

Integer satoshi rounding eventually makes the subsidy zero. At era 32 the shifted subsidy is 1 satoshi; at era 33 it is 0. The implementation also explicitly guards large shift counts. This is why the familiar 21 million BTC ceiling is a cap description, rather than a promise that every fraction below that number will be issued and spendable. Lost coins and unclaimed subsidy are separate questions.