Ethereum's unit system
Ether subdivides by powers of ten: 1 ETH = 1,000,000,000 gwei (109) and 1 gwei = 1,000,000,000 wei, so 1 ETH = 1018 wei. Wei — named for cryptographer Wei Dai — is the base unit all smart contracts compute in. Gwei (giga-wei) exists because it's the convenient scale for gas prices: saying '20 gwei' beats saying '0.00000002 ETH'.
How a gas fee is actually calculated
Every operation costs a fixed number of gas units — 21,000 for a simple ETH transfer, around 65,000 for an ERC-20 token transfer, and 150,000+ for swaps and DeFi interactions. Your fee is gas units × gas price: at 20 gwei, a simple transfer costs 21,000 × 20 = 420,000 gwei = 0.00042 ETH, about $1.26 with ETH at $3,000. Complexity sets the units; network congestion sets the price.
Base fee, priority fee, and why gas prices swing
Since EIP-1559, the gas price you pay is a protocol-set base fee (burned) plus a small priority tip to the block producer. The base fee adjusts every block with demand, which is why gwei can sit in single digits on a quiet weekend and spike 10x during an NFT mint or market crash. Timing flexible transactions for off-peak hours is the simplest gas saving there is.
How to use this converter
Type a value into the wei, gwei, or ETH field to convert live between the three. For a real fee estimate, use the gas section: enter the gas units your transaction needs (21,000 for a plain transfer), the current gas price in gwei, and set the ETH price field to today's market. The tool multiplies them into both an ETH cost and a dollar cost.
A worked example
A Uniswap swap that consumes 150,000 gas units at a gas price of 25 gwei costs 150,000 × 25 = 3,750,000 gwei = 0.00375 ETH. With ETH at $3,200, that's about $12. Drop the gas price to 8 gwei on a quiet night and the same swap costs 0.0012 ETH, roughly $3.84 — the identical transaction at a third of the price, purely because the network was less congested.