Every time you send crypto, swap a token or use an app on a blockchain, you pay a fee. This article explains what that fee is made of, why it changes from minute to minute, and how to read the numbers you see in a wallet or on the live gas page.
The word "gas" comes from Ethereum, but the same idea exists on every chain. Only the names and the units differ.
What gas is
Gas is a unit that measures how much work a transaction asks the network to do. Sending plain ETH from one address to another is a small job. Calling a smart contract that moves tokens, or swaps one token for another, is a bigger job. Bigger jobs use more gas.
Think of gas like kilowatt-hours on an electricity bill. The number of units tells you how much you used. The price per unit tells you what each unit costs right now. The fee is the two multiplied together:
fee = gas used x price per unit of gas
The gas amount depends on what the transaction does. The price depends on how busy the network is.
Gas limit and gas used
Before sending, your wallet sets a gas limit. This is the maximum amount of work you allow for the transaction. A node cannot know exactly how much work a contract call needs until it runs it, so the wallet makes an estimate and usually adds a safety margin.
After the transaction runs, the network records gas used. This is the real amount. You pay for gas used, not for the limit. If you set a limit of 100,000 and the transaction used 65,000, you pay for 65,000 and the rest is not charged.
If the limit is too low, the transaction runs out of gas and fails. The work done up to that point is still paid for. See how to read a transaction for how this looks on a transaction page.
Gwei and the price of gas
The price of one unit of gas is quoted in gwei. One gwei is one billionth of one ETH (0.000000001 ETH). Using gwei avoids long strings of zeros.
Example: a plain ETH transfer uses 21,000 gas. If the price is 10 gwei, the fee is 21,000 x 10 = 210,000 gwei, which is 0.00021 ETH. To see it in dollars, multiply by the current ETH price. That is why the same transaction can cost different dollar amounts on different days even when gas used stays the same.
EIP-1559: base fee, tip and max fee
Since the London upgrade, Ethereum prices gas with EIP-1559. The price per gas has two parts.
- Base fee. Set by the protocol, not by users. It rises when the previous block was more than half full and falls when it was less than half full. The base fee is burnt, which means it is destroyed and goes to nobody.
- Priority fee (tip). An optional extra you pay to the block producer. A higher tip makes your transaction more attractive when many people compete for space.
You also set a max fee: the highest total price per gas you accept. Your wallet pays base fee plus tip, as long as that is not above the max fee. If the base fee ends up lower than expected, you get the difference back. The max fee is only a ceiling, not what you pay.
The gas page uses the same idea. For each speed it shows a tip and a max fee, and the max fee is set to twice the current base fee plus the tip. The dollar costs it shows use base fee plus tip, because that is what a transaction actually pays.
Why different actions cost different amounts of gas
Here are typical amounts. They are rough, and a real transaction can differ.
| Action | Typical gas | Why |
|---|---|---|
| Send ETH to an address | 21,000 | Fixed cost of a simple transfer |
| Send an ERC-20 token such as USDC | about 50,000 to 65,000 | A contract updates two balances |
| Swap on a decentralized exchange | about 100,000 to 200,000 or more | Several contracts run, often with several token moves |
A simple ETH transfer is always exactly 21,000 gas. A token transfer is a smart contract call, so the network must run code and write to storage. A swap calls a router, one or more pools and token contracts, so it needs much more.
On the gas page, the "Send coin", "Send token" and "Swap" choices use 21,000, 65,000 and 180,000 gas as reference sizes. They are typical values, not a measurement of your own transaction.
Economy, normal and fast
Wallets and the gas page offer three speeds. The difference is the tip.
- Economy: a small tip. It usually gets in, but may wait when the network is busy.
- Normal: a typical tip, matching what recent blocks paid.
- Fast: a higher tip, for when you want to be early in the next block.
On the gas page, these tips come from recent blocks: the 10th, 50th and 90th percentile of the tips paid over the last 20 blocks. Not every chain has a tip market. Arbitrum ignores tips, so all three speeds cost the same there.
Fees in US dollars
Gas prices are in gwei, but people think in dollars. To show a dollar fee, a tool multiplies the fee in the native coin by the coin's price. The gas page does this for seven chains and lets you compare them. On Ethereum layer 2 networks, the figure also includes the data fee paid to Ethereum. Why fees differ between chains explains that part.
The same idea on Bitcoin and Solana
Bitcoin has no gas. Its fee depends on the size of the transaction, measured in virtual bytes (vB). The price is quoted in satoshis per vB (sat/vB). A satoshi is one hundred-millionth of a bitcoin. A typical simple payment is around 140 vB, but it grows with more inputs and outputs. Read more in Bitcoin mempool and fees.
Solana measures work in compute units. Each transaction pays a small fixed fee per signature, counted in lamports (one billionth of a SOL). It can also pay an optional priority fee to be processed sooner. See Solana priority fees.
How to keep fees low
- Send when the network is quiet. Fees are often lower when fewer people are active.
- Pick the Economy speed if you are not in a hurry.
- Use a cheaper chain when the receiver can accept it. See the cheapest way to send USDC.
- Check the live price before you confirm, not after.
A very low fee can leave a transaction waiting for a long time. It does not lose your funds, but it may stay pending until the price drops or you replace it.
Frequently asked questions
What is gas in crypto?
Gas is a unit that measures how much computing work a transaction needs on a blockchain. You pay a price for each unit, and the total is your fee. More complex actions use more gas.
What is a gwei?
A gwei is one billionth of one ETH. It is the unit used to quote the price of gas. A price of 20 gwei means 0.00000002 ETH for each unit of gas.
Why does sending USDC cost more than sending ETH?
A plain ETH transfer is a simple balance change and uses 21,000 gas. A USDC transfer calls a token contract, which runs code and updates stored balances. That takes more gas, often about 50,000 to 65,000.
Do I pay the max fee?
No. The max fee is a ceiling. You pay the base fee plus your tip, and any unused part of the max fee is not charged.
Why is my transaction fee different from the estimate?
The base fee changes with every block, and the gas used can differ from the wallet's estimate. Bigger swings happen when the network suddenly gets busy.
Where can I see current gas prices?
Open the live gas page to see economy, normal and fast costs on all seven chains. For a single chain, open its page such as Ethereum.