Gwei is short for “giga-wei”. Wei is the smallest unit of ether — one ether equals 10^18 wei — and gwei is one billion wei, or 10^-9 ether. It is the unit that gas prices are almost always quoted in, because it is a convenient size: gwei values are usually small integers or low-hundreds, which is much easier to read than either “0.00000002 ETH” or “20000000000 wei”.
When your wallet says the gas price is “25 gwei”, it means 25 × 10^-9 ETH per unit of gas. A transaction using 21,000 gas (the cost of a basic transfer) at 25 gwei would cost 21,000 × 25 × 10^-9 = 0.000525 ETH. At an ETH price of $3,000, that is about $1.58. Most Ethereum wallets do this math for you and show the total in USD, but if you want to understand what is being charged, the gwei value is the honest number.
The History of Gas Prices in Gwei
Ethereum’s gas prices have moved around a lot. In the early years (2015-2016), 20 gwei was a normal busy-day price. During the 2017 ICO boom, prices sometimes spiked to 500+ gwei for a few hours as people rushed to mint tokens. DeFi summer in 2020 pushed typical prices into the 50-200 gwei range for months on end. NFT mint events regularly hit 1,000+ gwei at the peak. After EIP-4844 in March 2024 moved rollup settlement off mainnet, typical prices have settled into the 1-20 gwei range most of the time, with occasional spikes during volatility.
If you are watching Ethereum gas prices, sites like etherscan.io/gastracker show real-time estimates broken into tiers (low, average, fast). Wallets like MetaMask pick a price for you based on similar data, and you can usually override it manually if you want to pay less and wait longer, or more and get included sooner. Under EIP-1559 the math is a little more complex — there is a base fee plus an optional tip — but the units are still gwei and the mental model still works.
Why Not Just Use ETH
You could. The reason nobody does is that gas prices in ETH look awful. “0.00000002 ETH per gas” is hard to parse at a glance. “20 gwei” is not. Every unit in the wei ladder exists for a specific use case where that unit is the right size for human reading, and gwei is the sweet spot for gas prices.
The other units you might see: wei (the base), kwei (10^3), mwei (10^6), gwei (10^9), szabo (10^12), finney (10^15), and ether (10^18). The last three are mostly historical — szabo and finney were named after cryptographers Nick Szabo and Hal Finney, but nobody actually uses them in practice. Gwei and ether are the two units you will encounter; everything else is trivia.