Wei, gwei and ETH
Ethereum's currency has one headline unit, ETH (ether), but under the surface the network only ever counts wei, its smallest unit, named after the cryptographer Wei Dai. One ETH is a full quintillion wei, so a middle unit gets constant use: the gwei, short for gigawei, one billion wei. Gas prices, the fees you bid to have a transaction processed, are always quoted in gwei.
Converting between them is only ever a matter of moving the decimal point: nine places between ETH and gwei, nine more between gwei and wei, eighteen in one jump between ETH and wei. Pick the unit you have, type the amount and the other two appear.
Worked examples
Why gas fees are quoted in gwei
Gas is Ethereum's measure of computational work: a plain ETH transfer costs 21,000 gas, and more complex contract calls cost more. What you pay is the gas used multiplied by a price per unit of gas, and that price lands in a range where gwei is the only readable unit. A price of 30 gwei would be 0.00000003 ETH or 30,000,000,000 wei; both are easy to misread by a factor of ten, while "30 gwei" is instantly comparable with yesterday's 12 or last bull market's 200.
Since the London upgrade, that gas price is made of a base fee that the network burns plus a priority tip to the validator, but both parts are still set in gwei, so the conversion habit stays the same: gas × gwei price, then shift nine places to read the fee in ETH.
Exact maths, no floating point
Ordinary JavaScript numbers are double-precision floats, safe to roughly 15 or 16 significant digits. One ETH expressed in wei is already a 19-digit number, so a converter built on floats will silently corrupt the tail of large wei amounts and sprinkle rounding dust over small ETH ones. This tool never touches floats: your input is parsed as text, converted to a BigInt count of wei, and every displayed value is produced by shifting the decimal point through that digit string. Whether you enter 1 wei or millions of ETH, every digit shown is exact.
Safety note: never enter your seed phrase or private keys into any website, including this one. A unit converter only ever needs numbers. This page runs entirely in your browser, fetches no prices and sends nothing anywhere.
Figures are for guidance only and are not financial advice. Cryptoassets are high risk and largely unprotected in the UK, so check FCA guidance before acting on any calculation.
Frequently asked questions
How many wei are in one ETH?
One ETH is 1,000,000,000,000,000,000 wei, which is 10 to the power of 18. One ETH is also 1,000,000,000 gwei, and one gwei is 1,000,000,000 wei. Wei is the smallest unit the Ethereum network records.
What is a gwei?
Gwei is short for gigawei, one billion wei, which makes it one billionth of an ETH. It is the unit gas prices are quoted in, because typical gas prices come out as tidy numbers like 20 or 30 gwei rather than unreadable strings of zeros in either wei or ETH.
How do I work out a gas fee in ETH?
Multiply the gas used by the gas price in gwei, then convert. A simple ETH transfer uses 21,000 gas, so at 30 gwei that is 21,000 × 30 = 630,000 gwei, which is 0.00063 ETH. Multiply that by your ETH price to see the fee in pounds.
Why does the converter use BigInt instead of normal numbers?
Ordinary JavaScript numbers are floating point and only safe to about 15 or 16 digits, but one ETH is already a 19-digit number of wei. This tool parses your input as text, converts it to a BigInt number of wei, and moves the decimal point by shifting digits, so every result is exact.
Is it safe to type amounts into this page?
The tool only ever needs plain numbers and runs entirely in your browser; nothing you type is sent anywhere and no prices are fetched. Never enter your seed phrase or private keys into any website, including this one. No legitimate converter needs them.