A satoshi is not another token or type of bitcoin. It is a denomination of the same asset: 1 sat = 0.00000001 BTC. Bitcoin Core represents amounts as integer CAmount satoshis; wallets may display BTC or sats. Lightning additionally accounts in millisatoshi, but on-chain settlement produces whole satoshis.
The fixed conversion is 1 BTC = 100,000,000 sat and 1 sat = 0.00000001 BTC. Moving the decimal eight places must be exact: 0.001 BTC is 100,000 sat and 25,000 sat is 0.00025 BTC. A fiat exchange rate does not alter this identity.
Bitcoin Core defines CAmount as a 64-bit integer and COIN as 100,000,000 satoshis. Consensus-sensitive arithmetic therefore avoids binary floating point and inconsistent rounding. Decimal BTC is a presentation layer.
BTC, sat and sats are scales of the same bitcoin asset. Conversion creates no coins, changes no private key and moves no UTXO. The name honors Satoshi Nakamoto but is not a special coin.
A transaction output contains a whole-satoshi value and a Script. One UTXO can hold one sat or many BTC; wallets usually sum several UTXO. A satoshi is not a physical serialised coin independently traced outside output history.
The subsidy is counted in whole satoshis and halving uses integer shifts until the result becomes zero. MAX_MONEY is a consensus-critical range check, not a promise that exactly 21 million BTC will be spendable. Lost keys further reduce accessible supply.
Recipient value is in outputs; transaction fee is inputs minus outputs. Paying 100,000 sat does not mean the fee is 100,000 sat. Change may return in another output, and a denomination error can cause a severe overpayment.
Fee rate is commonly quoted in sat/vB: satoshis per virtual byte. Total fee is roughly fee rate times vbytes, not a percentage of value. Equal-value transactions can cost differently because their inputs and outputs differ.
One sat is a valid protocol value, but every tiny output is not economical or standardly relayed. Dust threshold is node policy related to future spending cost and Script type; it is neither a universal consensus minimum nor sub-satoshi divisibility.
Lightning denominates HTLCs and some fees in millisatoshi: 1 sat = 1,000 msat. This is channel accounting, not an on-chain output below one sat. Closing transactions round to whole satoshis and may trim small outputs.
Check the interface unit, decimal places, recipient amount and fee separately. BIP 321 bitcoin URIs express amount in decimal BTC even when a wallet displays sats. Convert once to integer satoshis, avoid floating point and review outputs again before signing. Sources: Bitcoin Core — consensus amount.h; Bitcoin Developer Reference — Glossary: denominations and outputs; BIP 321 — URI Scheme; Lightning BOLT #1 — Base Protocol: amount types; Lightning BOLT #3 — Transactions: whole-satoshi settlement.
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