Satoshi

A satoshi(often shortened to “sat” in slang) is the smallest indivisible unit of Bitcoin. One bitcoin equals exactly 100,000,000 satoshi (10⁸). Named after Satoshi Nakamoto, the pseudonymous author of the 2008 Bitcoin whitepaper, whose true identity has never been definitively established despite years of public speculation.

Bitcoin protocol stores all values as integer satoshi amounts internally. The 21-million BTC supply cap maps to exactly 2,100,000,000,000,000satoshi (2.1 quadrillion). That fits comfortably inside a signed 64-bit integer (max ~9.2 quintillion), which is why Bitcoin wallet software historically didn’t need BigIntthe way Ethereum tooling does — Ethereum’s 18-decimal precision and large balances exceed 64-bit limits.

When you’ll see satoshi in practice

Block explorers (Mempool.space, blockchain.com) usually show BTC by default but offer a satoshi toggle. Wallet software lets you switch between BTC, mBTC (millibitcoin, 10⁻³), and satoshi as display units. Mining rewards and transaction fees are denominated in satoshi at the protocol level; user-facing UIs convert to BTC for readability when the amounts are large.

Sub-satoshi units (Lightning Network)

For Lightning Network channels — Bitcoin’s layer-2 payment network — millisatoshi (1 msat = 10⁻³ satoshi) is the unit. Lightning fees, especially on routing hops, are often sub-satoshi; the msat denomination lets fee accounting work precisely without losing fractional value. Millisatoshis only exist inside Lightning channels — they don’t settle on the base Bitcoin chain, which is integer-satoshi only.

Cultural notes

“Stacking sats” is Bitcoin community shorthand for steadily accumulating Bitcoin in small amounts over time, rather than trying to time the market. The word “sat” appears in exchange UIs (Strike, Cash App), educational content (Lightning tip jars), and merchandise. Some Bitcoin-pricing units are themselves satoshi-denominated: “sats per byte” (transaction fee rates) and “sats per vByte” (witness-discounted fees on SegWit transactions) are the operational fee units, not BTC.

Why it matters: a worked fee example

A typical Bitcoin transaction is around 140 vBytes (a single-signer P2WPKH spend with one input and two outputs). At a fee rate of 30 sat/vByte — typical 2024-2025 mempool conditions for confirmation within a few blocks — the total fee is 140 × 30 = 4,200 satoshi, or 0.000042 BTC. At a BTC price of $60,000 that is approximately $2.52 per transaction. During periods of mempool congestion the fee rate can spike to 200-500 sat/vByte (so the same transaction costs $17-$42), which is why fee estimators that report in sat/vByte rather than BTC are essential for users who care about cost. Lightning Network channel opens and closes are normal on-chain transactions that follow this same fee math; routed Lightning payments themselves are denominated in millisatoshi inside the channel and never touch the base chain.

The supply schedule, in satoshi

The Bitcoin block subsidy halves roughly every four years (every 210,000 blocks). Starting at 50 BTC per block (5,000,000,000 satoshi) in 2009, the subsidy dropped to 25 BTC at the 2012 halving, 12.5 BTC in 2016, 6.25 BTC in 2020, and 3.125 BTC (312,500,000 satoshi per block) after the April 2024 halving. The last satoshi will be mined around 2140, after which miners will be compensated solely by transaction fees. The integer-satoshi arithmetic ensures the 21-million cap holds exactly — there is no rounding error compounding across 33 halvings because every subsidy is a whole number of satoshi until the final blocks. References: Bitcoin: A Peer-to-Peer Electronic Cash System (Nakamoto, 2008), BIP 141 — Segregated Witness.