Denomination

Only two of these units mean anything to the software: the whole coin, used in the RPC interface and on exchange screens, and the satoshi, which is what the protocol counts. Everything in between is a display preference, the way a shop can quote 1.50 euros or 150 cents for the same price. Choosing a comfortable unit is not a trivial matter of taste, because misreading a decimal place is an expensive mistake.

How it works

Bitcoin uses eight decimal places, and every named denomination is a fixed slice of that.

One bitcoin is 1,000 millibitcoin, 1,000,000 microbitcoin, and 100,000,000 satoshis. The microbitcoin has an informal name, the bit, which BIP176 proposed in 2017 as a standard unit of 100 satoshis; the proposal never became a consensus rule and it competes with the ordinary computing meaning of the word, so adoption stalled. Meanwhile the satoshi won the popular vote, helped by the fact that a five figure satoshi amount reads like a normal price while 0.00042 BTC does not.

The ticker itself has two forms. Most exchanges and wallets write BTC, but Kraken, Bloomberg terminals, and several banks write XBT, because ISO 4217 reserves an initial X for currencies that no country issues, the same convention that gives gold the code XAU. BTC and XBT are the same unit; a wallet that shows one and an exchange that shows the other are not describing different assets.

Bitcoin Core lets you pick your display unit in the GUI, offering BTC, mBTC, microBTC, and satoshis, while its JSON-RPC interface always speaks in whole coins with eight decimals. That mismatch is worth remembering if you ever script anything: the human interface is configurable, the machine interface is not.

Where you see it

Denominations show up at exactly the moments when a mistake costs money.

Withdrawal screens are the first. An exchange will ask for an amount in BTC while your wallet displays satoshis, and 0.001 BTC, 1 mBTC, and 100,000 satoshis are all the same withdrawal. Slipping one decimal place turns a 100 dollar transfer into a 1,000 dollar one at a six figure price, which is why most wallets now show a fiat equivalent underneath the field. Read that line before confirming.

Fee interfaces are the second. Nobody quotes fees in bitcoin: they are quoted in satoshis per virtual byte, so a 141 vbyte withdrawal at 3 sat/vB costs 423 satoshis. Wallets that show fees in dollars are converting from that figure, and their conversion is only as fresh as their price feed.

Lightning invoices are the third. A Strike or Lightning-enabled wallet payment is denominated in satoshis, and its internal accounting goes finer still, down to millisatoshis. Nothing on that layer is priced in whole coins.

Finally, denomination shapes how people talk about buying. "Stacking sats" exists as a phrase because a recurring purchase of 20 dollars is a growing satoshi balance rather than a rounding error in a coin balance, and platforms built around recurring buys, such as Swan and River, present it that way deliberately. The unit is a psychological interface as much as a numeric one, which is fine, as long as you know exactly which one your screen is using before you press send.