Most of the time your wallet shows one small number, the network fee. But sooner or later you will open an advanced screen or a block explorer and see terms like "gas limit", "gas used" and "gwei". They follow a simple pattern, and once you see it, a fee becomes plain multiplication. If gas itself is new to you, start with our overview of gas fees on Base first.
Gas is a unit of work, not a coin
The word "gas" is used in two slightly different ways, which causes most of the confusion.
The first meaning is a unit of work. Every step a transaction asks the network to perform has a fixed cost measured in gas units. Sending ETH from one address to another is the simplest action, and on Ethereum-compatible networks it costs 21,000 gas units. A token transfer costs more, a swap costs more again, and a complicated action that touches several smart contracts costs more still. Gas units have no dollar value by themselves. They only measure how much work something takes.
The second meaning is loose, everyday speech: "I paid gas" really means "I paid the fee." The fee is paid in ETH, not in gas units. Keeping the two ideas apart is the key to everything below.
Gwei is the price of one unit of gas
If gas units measure work, something has to set the price per unit. That price is quoted in gwei.
One gwei is one billionth of an ETH (0.000000001 ETH). The name is short for "giga-wei", and wei is the smallest ETH unit. Gwei is just a convenience, like quoting a small distance in millimeters instead of a tiny fraction of a kilometer.
So a gas price of 10 gwei means each unit of gas costs 10 billionths of an ETH. On Ethereum mainnet, gas prices are often quoted in whole gwei. On Base they are usually far smaller, often a small fraction of one gwei, which is a big reason fees there are so low.
The gas limit is a ceiling, not a bill
Before your wallet submits a transaction, it estimates how much work the transaction will need and sets a gas limit. This is the maximum number of gas units you are willing to let the transaction consume.
The limit protects you. If something goes wrong and the transaction tries to run far longer than expected, it is cut off at your ceiling instead of draining your balance. Wallets usually add a little headroom to the estimate, so the gas limit is often a bit higher than the gas actually used.
Gas used is what you actually pay for
When the transaction is confirmed, the network records gas used, the number of units the work really took. You pay for gas used, not for the gas limit. Any gas you set aside but did not need is returned to you.
Here is the whole calculation in one line:
Fee = gas used × gas price
Here is an illustrative example with round numbers. Say a simple ETH transfer uses 21,000 gas units and the gas price is 10 gwei:
- 21,000 × 10 = 210,000 gwei
- 210,000 gwei = 0.00021 ETH
The same arithmetic works on Base, only with a much smaller gas price. Suppose a swap uses 150,000 units, a made-up figure for illustration, and the gas price is 0.01 gwei. That gives 150,000 × 0.01 = 1,500 gwei, which is 0.0000015 ETH. Real swaps vary widely, so treat these numbers as practice, not a quote.
Where the base fee and priority fee fit
The gas price is not always a single number. Networks that use the EIP-1559 fee model split it into a base fee, set by the protocol, and a priority fee, an optional tip. Wallets often also show a max fee, the highest price per unit you are willing to pay. If the actual base fee plus tip comes in below your max fee, the difference is refunded. Our guide to the base fee, priority fee and L1 data fee goes through that split in detail.
Base has one extra line: the L1 data fee
Because Base is a rollup that posts its data to Ethereum, every Base transaction carries a second cost on top of execution. Base's documentation describes the total as an L2 execution fee plus an L1 security fee, and notes that the L1 part is typically the larger of the two and moves with activity on Ethereum.
The gas math above only explains the execution part. Your wallet adds the L1 data fee behind the scenes, so the figure it shows is the sum. If your own multiplication does not match exactly, this is the usual reason.
What happens when a transaction fails
A failed transaction can still cost you money, and this surprises many people. If a transaction runs out of gas partway through, or the contract rejects it after some work has already been done, the network has still spent effort on it. Ethereum's documentation states that in the out-of-gas case, all the gas provided is still consumed for the work performed. A transaction that is rejected before it is included in a block, on the other hand, consumes no gas.
Setting a gas limit too low by hand is an easy way to pay for a transaction that does nothing. For the usual causes of failures, see why a transaction fails on Base.
Reading the numbers on a block explorer
On a block explorer such as BaseScan, the transaction page lists the gas limit, the gas used, the gas price and the total fee. Gas used can never exceed the gas limit, and gas used times the gas price should roughly match the execution part of the fee. On Base the total can be higher because of the L1 data fee. Our walkthrough on using a block explorer shows where each field lives.
Habits worth keeping
- Leave the gas limit to your wallet. The automatic estimate is almost always the right choice, and manual edits rarely save meaningful money on Base.
- Keep a small ETH balance on Base. Every action needs ETH for the fee, even when you are swapping other tokens.
- If a transaction gets stuck, see how to speed up or cancel it.
The short version
Gas units measure how much work a transaction needs. Gwei is the price of one unit. The gas limit is the most you allow, gas used is what was actually needed, and the fee is gas used times price, with Base adding an L1 data fee on top. Once those terms are clear, the numbers in your wallet are far less intimidating.
This article is educational and is not financial advice. Fee levels change with network conditions, so always check the figure your wallet shows before you confirm.