A satoshi is the smallest unit of Bitcoin — one hundred millionth of a BTC, or 0.00000001 BTC. The name is a tribute to Bitcoin’s pseudonymous creator, Satoshi Nakamoto, and has been in use among Bitcoin users since at least 2010. Everyone in the Bitcoin ecosystem calls them “sats” in casual conversation, and the unit has become standard enough that prices for small Bitcoin-denominated things (Lightning payments, Ordinals inscriptions, small on-chain fees) are usually quoted in sats rather than in BTC.
The technical reason sats exist is that Bitcoin’s protocol internally tracks all balances and transaction amounts in satoshis rather than in BTC. The conversion to BTC is purely for human display; at the database level, your 0.5 BTC is actually stored as 50,000,000 satoshis. Because Bitcoin’s maximum supply is capped at 21 million BTC, the total number of satoshis that will ever exist is 21 × 10¹⁴, or 2.1 quadrillion. That is a large enough number that sats will remain practical units for small-value transactions even if Bitcoin’s price reaches values that make whole-BTC units unwieldy.
The “Sats” Culture
The Bitcoin community has leaned into sat-denominated pricing for cultural reasons as much as technical ones. One of the arguments Bitcoiners make against the “Bitcoin is too expensive at $X per coin” mental model is that fractional ownership is always possible and the unit of account can scale with the price. If BTC is worth $100,000 and you own 100,000 sats, you own $100 worth of Bitcoin, and nothing about the fractional amount makes the ownership less meaningful than owning one whole unit of something cheaper. Quoting prices in sats reinforces this — “this coffee costs 1,500 sats” feels more natural for small-value transactions than “this coffee costs 0.000015 BTC”, and the former is more likely to stick in people’s heads as the unit of price they actually think in.
This is the rationale behind campaigns like “stack sats” (the Bitcoin community’s version of dollar-cost averaging, where the goal is to accumulate satoshis gradually) and behind the push to label prices in sats on Bitcoin-native services. Whether the unit labeling actually matters for long-term adoption is unclear, but it is a real cultural marker.
Sats in Lightning and Ordinals
The two places satoshis show up most visibly in day-to-day use are Lightning Network payments and Ordinals inscriptions.
Lightning uses millisatoshi (msat) as its internal unit — one thousandth of a sat — because Lightning supports much smaller payments than the Bitcoin base layer, and whole-satoshi granularity was deemed too coarse for some use cases. Lightning invoices are usually denominated in sats for display but the actual routing calculations happen in msats. Most Lightning payments in practice are in the range of a few hundred to a few million sats (i.e., a fraction of a cent to a few tens of dollars), which is the sweet spot where Lightning’s cost advantage over the base layer is largest.
Ordinals (a Bitcoin NFT-like system introduced in 2023) give individual satoshis unique identities based on the order they were mined. A “rare sat” might be the first satoshi of a block, the first satoshi after a halving, or some other numerically distinctive position in the total issuance. Collectors trade these rare sats at premiums to their face value, treating them like collectible coin varieties. The Ordinals theory of sat rarity produced a niche market where individual satoshis are tracked and sold as unique items, which is a use case Satoshi almost certainly did not anticipate when designing the unit in the first place.
Other Names That Didn’t Stick
Over the years, other sub-BTC units have been proposed and occasionally used. “Bit” referred to one millionth of a BTC (100 sats) and appeared in some wallets around 2014, but never achieved meaningful adoption. “uBTC” (micro-BTC, the same as “bit”) shows up occasionally in technical documentation but not in user-facing interfaces. “mBTC” (milli-BTC, one thousandth of a BTC) is slightly more common and is still supported by some wallets as a display option. None of these have the cultural weight of “sat”, and the Bitcoin community has essentially settled on sats as the secondary unit after BTC itself, with everything else being trivia.