Wei

Wei is the base unit of ether. All balances and values on Ethereum are stored internally as whole numbers of wei, and wallets convert them to ETH for display. One ETH is 10 to the 18th power wei, written as 1 followed by 18 zeros. The unit is named after Wei Dai, a cryptographer whose 1998 b-money proposal influenced Bitcoin.

Because the number is so large, intermediate units are used in practice. One gwei is 1 billion wei and is the standard unit for gas prices. Developers working with smart contracts deal in wei directly to avoid rounding errors, since the Ethereum Virtual Machine has no decimal numbers.

The 18-decimal convention carries over to most ERC-20 tokens, which also store balances as integers with 18 decimal places, although some tokens such as USDC use 6 decimals. Bitcoin's equivalent smallest unit is the satoshi, which is 1 hundred-millionth of a bitcoin, a far smaller divisor.