How transaction fees work

What a fee is #

A fee pays the miner to include your transaction in a block, and it discourages spam. On UTXO chains like Bitcoin and Litecoin there's no separate "fee" field. The fee is whatever's left over: inputs − outputs. Anything you don't assign to an output goes to the miner. (This is why understanding UTXOs helps.)

Fee rate vs total fee #

Miners care about the fee rate, the fee per unit of transaction size, priced in satoshis per virtual byte (sat/vB) on Bitcoin and Litecoin. Your total fee is the rate times your transaction's size. A transaction that spends many small inputs is bigger, so it costs more to send at the same rate.

The mempool #

Broadcast transactions wait in the mempool, a holding area of unconfirmed transactions. When a miner builds a block, it picks the highest fee rates first because they earn the most. If the network is busy, low-fee transactions wait; when it's quiet, even a low fee confirms quickly.

Fee estimation #

Wallets estimate a fee rate to hit a target confirmation time by looking at what's currently in the mempool. Pay more to confirm sooner, less to save money and wait longer. See how confirmations work.

Fees on testnet #

Testnet transactions follow the same fee rules, but since the coins are free, it costs nothing to experiment. Send the same payment at a high and a low fee rate and watch how much sooner one confirms. It's the cheapest way to build real intuition.

Monero fees #

Monero handles fees a bit differently: rather than a sat/vB market, you choose a priority level and the protocol sets a dynamic fee. Higher priority pays more for faster inclusion. See the Monero guide.