Bitcoin's code does not use decimals. Every amount is stored as a whole number of satoshis. In Bitcoin Core, one coin is defined as COIN = 100,000,000.
The units
- 1 BTC = 100,000,000 sats
- 1 mBTC (millibitcoin) = 0.001 BTC = 100,000 sats
- 1 bit (μBTC) = 100 sats, proposed in BIP176
- 1 sat = 0.00000001 BTC, the smallest on-chain unit
- 1 msat = 1/1000 sat, used inside the lightning network; on-chain it rounds away
Why sats
At a price of roughly $81,000 per BTC (23 October 2026), $1 is about 1,230 sats. Saying "1,230 sats" is easier than "0.0000123 BTC" and makes small payments easier to read.
It also helps with the "I can't afford one bitcoin" misunderstanding: you can buy any amount down to a few sats.
How many sats exist
The protocol caps the supply at just under 21 million BTC, about 2.1 quadrillion sats. The code's sanity limit, MAX_MONEY, is exactly 21,000,000 × COIN; the real issued total ends slightly lower because of rounding in the halving schedule.
Fees are in sats too
Fee rates are quoted in sats per virtual byte (sat/vB). A typical simple payment is around 140 vB, so at 2 sat/vB it costs about 280 sats.
Takeaway
One bitcoin, one hundred million sats. Thinking in sats makes bitcoin practical for everyday amounts.