Wei refers to the smallest denomination of ether (ETH), the currency used on the Ethereum network. For instance, 1 ether (ETH) is equivalent to 1×1018 wei (which may also be represented in exponential notation: 1e18 wei).
As the price of Ether and other cryptocurrencies rises, the size of each blockchain transaction tends to become smaller. For example, if ETH is priced at $10, one would need 10 ETH to make a $100 transfer. But if ETH is priced at $150, only 0.67 ETH would be sufficient to transfer the same value in USD. Using ether denomination may be troublesome as the transaction size becomes smaller.
In this context, wei and other denominations of ether may be useful for describing small value transactions, especially in regards to very small amounts of ETH that are typically used for transaction fees or when setting the gas limit of a transaction.
As presented on the Ethereum technical documents, there are several other denominations of ether, each representing a multiple of wei. Gwei and ether are among the most used denominations.
| Unit | Wei Value | Wei | Ether Value |
| Wei | 1 wei | 1 | 10-18 ETH |
| Kwei | 10³ wei | 1,000 | 10-15 ETH |
| Mwei | 106 wei | 1,000,000 | 10-12 ETH |
| Gwei | 109 wei | 1,000,000,000 | 10-9 ETH |
| Microether | 1012 wei | 1,000,000,000,000 | 10-6 ETH |
| Milliether | 1015 wei | 1,000,000,000,000,000 | 10-3 ETH |
| Ether | 1018 wei | 1,000,000,000,000,000,000 | 1 ETH |
Similarly, Bitcoin uses the term satoshi to describe the smallest unit of its currency. 1 satoshi is equivalent to 0.00000001 BTC (or 10-8 BTC).
