What is Gas fees?

How it works

Every action on a blockchain — a swap, a token transfer, deploying a contract, even just registering an ENS name — has a computational cost. Validators pick which pending transactions to include in the next block. To compete for limited block space, users attach a fee. Higher fee = faster inclusion.

On Ethereum the fee is denominated in "gas," with each operation costing a fixed number of gas units. The actual dollar fee is gas units × gas price × ETH/USD price. A simple ETH transfer is 21,000 gas; a Uniswap swap is around 100,000–200,000; a complex DeFi position can run 500,000+.

Example

A Uniswap swap on Ethereum mainnet during a quiet weekend morning at a 5 gwei gas price with ETH at $3,400:

150,000 gas × 5 gwei × 10⁻⁹ × $3,400 = $2.55

Mid-week during a heavily-traded NFT mint, the same swap at 100 gwei:

150,000 gas × 100 gwei × 10⁻⁹ × $3,400 = $51

Same trade, 20× the cost — purely because the network was congested. The price of the asset you're swapping doesn't change the gas; only the network demand does.

Why it matters

Gas fees explain a lot of how crypto users actually behave:

• Layer-2 adoption. Arbitrum, Optimism, Base, and zkSync exist specifically to give Ethereum-style smart contracts at fees of $0.01–0.50 instead of $5–50. Most retail DeFi activity has moved there.
• Solana traffic. Sub-cent fees are why high-frequency activities (memecoin trading, on-chain games, NFT mints) gravitate to Solana even when the underlying app could run elsewhere.
• DeFi profitability math. A "10% APY" yield farm isn't actually 10% if entering and exiting costs $80 in gas — you need a position size and time horizon where the fees amortize.
• The "stuck transaction" problem. Submitting a tx with too low a gas price means it sits in the mempool indefinitely. Most wallets let you "speed up" by replacing it with a higher-fee version.

Before any DeFi move, check current gas on a tracker like etherscan.io/gastracker or solscan.io. A 30-minute wait for off-peak gas can save more money than the trade itself makes.