Wei is a unit for expressing ETH amounts, not a separate cryptocurrency. Ethereum represents native balances and transaction values using whole numbers of wei. This allows very small amounts to be recorded precisely without treating a displayed decimal fraction of ETH as a different asset.
Converting the units
One ETH equals 1,000,000,000,000,000,000 wei. One gwei equals 1,000,000,000 wei, so one ETH also equals one billion gwei. For example, 0.01 ETH is 10,000,000,000,000,000 wei.
These are fixed denomination conversions. They do not depend on ETH’s market price, exchange rates, or network congestion. A wallet may round the displayed ETH amount while the underlying recorded value remains more precise.
Wei and transaction fees
Gas measures computational work; wei measures an amount of ETH. A gas price expressed in wei per gas must be multiplied by gas used to obtain an execution fee in wei. The gas quantity itself is not a currency balance.
Software must keep units and integer precision consistent when reading balances or preparing transactions. Confusing wei with gwei or ETH can create enormous amount errors. ERC-20 tokens have their own decimal conventions, so their smallest units should not automatically be called wei.