BTC, mBTC and satoshis: the units bitcoin is divided into

The satoshi is the unit the ledger uses, and that has consequences. An output's value is an integer number of satoshis, and the protocol cannot represent a fraction of one. The block subsidy halves every 210,000 blocks, and eventually the halving produces a value smaller than one satoshi. At that point the subsidy rounds to zero and the issuance stops, which is why the supply converges on twenty-one million rather than reaching it exactly. The supply schedule page sets out that arithmetic.

The base unit also sets the smallest possible on-chain payment. A transaction output cannot carry less than one satoshi, and an output whose value is smaller than the fee required to spend it is economically unspendable. That is the origin of the dust threshold discussed on the dust transactions page. The unit is not merely a display choice; it is a hard floor on what the ledger can express.

A third consequence is precision in accounting. Because amounts are integers, there is no rounding error in the protocol's own arithmetic. A wallet that displays a balance in bitcoin is converting from an exact integer to a decimal for the reader's benefit, and the conversion is exact because the divisor is a power of ten. This is a small but real advantage over systems that represent balances as floating-point numbers.

The unit a price is quoted in changes how a reader perceives it, and that is a practical matter rather than a cosmetic one. A price expressed as a small decimal fraction of a bitcoin is difficult to compare at a glance with a price expressed in whole units. This is why some wallets and merchants quote in satoshis or in bits: the numbers become integers, and integers are easier to reason about.

The choice of unit does not change the underlying value. A payment of 100,000 satoshis is a payment of 0.001 BTC is a payment of 1 mBTC. The three statements are identical, and a reader who prefers one denomination over another is expressing a preference about presentation rather than about the asset. The protocol is indifferent to which unit a wallet chooses to display.

There is one place where the choice is not merely presentational, and it is worth stating. Because the ledger stores satoshis, a wallet that displays bitcoin must round when it shows a balance, and the rounding must be done carefully to avoid showing a spendable balance that is larger than the actual one. A wallet that displays eight decimal places is showing the exact figure; a wallet that displays fewer is showing an approximation. The distinction matters when a user is trying to spend the last of a balance.

The units also matter for how the asset is described in prose. A writer who says "bitcoin is divisible" is making a claim about the protocol's arithmetic, and the claim is true up to the satoshi and no further. A writer who says "bitcoin can be divided infinitely" is making a claim that is false. The base unit is the boundary of the asset's divisibility, and it is a fixed boundary rather than a convention that could be changed by a wallet.