Master Ethereum Unit Conversions with Web3: From Wei to Ether and Tokens

Ethereum Unit System

The base unit of Ether is wei. One Ether equals 10^18 wei. Common denominations are: wei – 1 wei Kwei (babbage) – 1 000 wei Mwei (lovelace) – 1 000 000 wei Gwei (shannon) – 1 000 000 000 wei microether (szabo) – 1 000 000 000 000 wei milliether (finney) – 1 000 000 000 000 000 wei ether – 1 000 000 000 000 000 000 wei

Hexadecimal ↔ Decimal Conversion in Geth

After attaching to a running Geth node (e.g., geth attach /path/to/geth.ipc), use the following Web3 helpers:

> web3.toDecimal('0x16');
22

> web3.fromDecimal('22');
"0x16"

BigNumber Handling

Large integers that exceed JavaScript's safe range should be wrapped in a BigNumber to preserve precision (up to 20 decimal places).

> web3.toBigNumber('200000000000000000000001');
2.00000000000000000000001e+23

Values with more than 20 fractional digits are truncated.

Ether ↔ Wei Conversions with Web3.js

Use web3.fromWei(value, unit) to convert from wei to a higher unit, and web3.toWei(value, unit) for the opposite direction.

> web3.fromWei('22000000000000', 'ether');
"0.000022"

> web3.fromWei('1000', 'kwei');
"1"

> web3.fromWei('1000000000', 'gwei');
"1"

> web3.toWei('1', 'ether');
"1000000000000000000"

Sending a Transaction with Unit Conversion

eth.sendTransaction({
  from: eth.coinbase,
  to: "0x2bda4364bb076187f0ef0067a61ccb95d636e383",
  value: web3.toWei('1', 'ether')
});

Balances are returned in wei:

> web3.eth.getBalance(eth.coinbase);
267999999999999999999

ERC‑20 Token Decimals

In an ERC‑20 contract the decimals field defines how many smallest token units represent one whole token. For example, decimals = 3 means that 1 token = 1 000 smallest units.

uint8 public decimals;