Crypto transaction fees explained: what you pay and why

Why do crypto transactions have fees at all?#

A block can only hold so much. Bitcoin blocks are capped at 4 million weight units ^[3], and Ethereum blocks have a gas limit ^[2]. When more people want to transact than fit, the network needs a way to choose — and fees are that way. They also make spam costly: ethereum.org explains that charging for every computation prevents bad actors from flooding the network ^[2].

Fees also pay the people who secure the chain. The Bitcoin whitepaper describes fees as part of the incentive paid to whoever creates a block, and expects that once all new coins have been issued, the incentive can come entirely from fees ^[4]. You can see this reward split in our Bitcoin halving guide.

How is a Bitcoin fee calculated?#

Bitcoin has no “fee” field. The fee is whatever is left over: the value of all inputs minus the value of all outputs, paid to the miner who includes the transaction ^[5]. Your wallet sets it by deciding how much change to send back to you — see how Bitcoin transactions work for a worked example.

What you are really buying is space. The developer guide says fees are based on the byte size of the signed transaction, that fees per byte rise with demand for block space, and that miners fill blocks with the highest fee-per-byte transactions first ^[1]. Since the SegWit upgrade, size is measured in virtual bytes (vB): a transaction’s weight is its base size × 3 plus its total size, and its virtual size is that weight ÷ 4, rounded up ^[6]. Signature data (the “witness”) therefore counts for less than the rest.

Notice that the amount being sent never appears. A transaction moving a tiny amount with many inputs can cost more than one moving a large amount with a single input.

How is an Ethereum fee calculated?#

Ethereum charges for computation, measured in gas. Gas is the unit for the computational effort an operation needs, and the fee is the gas used multiplied by the price per unit of gas, paid in ether and usually quoted in gwei, one-billionth of an ETH ^[2]. Since the EIP-1559 upgrade the price has two parts: a base fee set by the protocol, which is burned, and a priority fee (tip) chosen by you, which goes to the validator ^[7].

If Jordan had set a gas limit of 50,000 for this simple transfer, only 21,000 would be used and the unused 29,000 returned ^[2].

Two Ethereum details catch beginners out. First, the fee is paid whether the transaction succeeds or fails; if a contract call runs out of gas partway, the changes are reverted but the gas is still consumed ^[2]. Second, complex actions such as token swaps use far more gas than a simple transfer, so the same network conditions can produce very different fees. Our Ethereum gas fees guide goes deeper.

Bitcoin from the bitcoin.org developer guide and BIP 141; Ethereum from ethereum.org and EIP-1559.

Why do fees go up and down?#

Demand. On Bitcoin, wallets compete in an open auction for the next block; when the mempool is crowded, the fee rate needed to get in rises. The Bitcoin Wiki puts it simply: the fee affects the likelihood of inclusion, and higher-fee transactions get priority ^[8]. Supply can move too. Coin Metrics notes that when many miners switched off during China’s 2021 mining ban, blocks came more slowly until the next difficulty adjustment and the mean fee per byte spiked ^[9].

Ethereum makes the base fee follow demand automatically. Each block has a target size of half its gas limit; if a block is fuller than the target, the base fee for the next block rises, and if it is emptier, it falls, by up to 12.5% per block ^[2]. EIP-1559 sets this through a change denominator of 8 and an elasticity multiplier of 2 ^[7].

Ethereum base fee after consecutive full blocks

Base fee (gwei), starting at 100

After six full blocks in a row, the base fee has roughly doubled. That steep climb is deliberate: ethereum.org notes that it makes long runs of full blocks economically unattractive ^[2]. On a busy day, the price of the same transfer can therefore change noticeably within minutes on either network.

What happens if you set the fee too low?#

On Bitcoin, the transaction waits in mempools and may take a long time to confirm, or expire. If it signalled Replace-by-Fee, you can broadcast a replacement that pays more; BIP 125 requires the replacement to pay at least the original fee plus the cost of its own size at the node’s minimum relay rate ^[10]. A second Bitcoin option is Child Pays For Parent (CPFP): spend an output of the stuck transaction in a new child transaction with a high fee rate, which encourages miners to include both ^[11]. On Ethereum, a transaction that offers only the base fee is technically valid but unlikely to be picked, because it gives validators no reason to choose it ^[2].

1. Check the current rate

   Most wallets estimate a fee for you. Fee estimation means working out the fee rate a transaction needs for a high probability of confirming within a chosen number of blocks ^[12] — it is an estimate, not a promise. Compare a few fee trackers if it matters.

2. Decide how urgent it is

   Paying for the next block costs more than accepting a wait of a few hours.

3. Prefer wallets with fee bumping

   RBF or CPFP on Bitcoin, or a speed-up option on Ethereum, lets you fix a fee that turned out too low.

4. Avoid many tiny inputs

   On Bitcoin, spending lots of small UTXOs makes a transaction bigger and therefore more expensive.

What mistakes do beginners make here?#

• Thinking fees are a percentage of the amount

  Network fees depend on size (Bitcoin) or gas (Ethereum), not on how much value moves. Sending 10 times more usually costs the same.

• Confusing an exchange’s withdrawal fee with the network fee

  A platform can charge whatever it likes for withdrawals. Only the on-chain part goes to miners or validators.

• Setting the lowest possible fee for an urgent payment

  A minimum fee can mean a long wait. Pay for the speed you actually need.

• Forgetting that failed Ethereum transactions still cost gas

  If a contract call fails, the gas already used is gone. Check the details before you confirm.

Frequently asked questions#

The bottom line#

A transaction fee is the price of a place in the next block. Bitcoin sets it by size in virtual bytes; Ethereum by gas used times a base fee that the protocol moves up and down, plus your tip.

Neither depends on the amount you send, and both swing with demand. Pick a fee that matches your urgency, use wallets that can bump a fee, and see how waiting transactions are queued in what is the mempool.

Sources#