Bitcoin Fees & Mempool

Every Bitcoin transaction includes a fee. The fee is the difference between the value of the inputs and the value of the outputs. Miners collect these fees, along with the block subsidy, when they include a transaction in a block. Fees serve two purposes. They compensate miners for the work of securing the network, and they create a market for limited block space. When demand for transactions exceeds the capacity of the next block, users who attach higher fees are more likely to be confirmed sooner. There is no fixed fee schedule. The market clears based on current demand and the fee rates users are willing to pay.

The mempool is the set of valid, unconfirmed transactions that a node has received but that have not yet been included in a block. Each full node maintains its own mempool. Transactions enter the mempool after they are broadcast and pass basic validity checks. Miners select transactions from the mempool, typically preferring those with higher fee rates (satoshis per virtual byte). Once a transaction is included in a block and that block is accepted by the network, it is removed from the mempool. Transactions that remain unconfirmed for a long time may eventually be dropped by nodes if they fall below minimum fee thresholds or if the mempool becomes full.

Fee estimation is the process of predicting how high a fee must be to achieve confirmation within a desired time. Wallet software and block explorers publish estimates based on recent blocks and the current mempool. These estimates are not guarantees. A sudden surge in demand can raise the required fee rate, and a quiet period can lower it. Users who need urgent confirmation can attach a higher fee or use techniques such as replace-by-fee (RBF) to increase the fee of an unconfirmed transaction. Users who can wait may choose a lower fee and accept a longer confirmation time.

Block space is scarce by design. The block size limit and the witness discount constrain how much data can be included in each block. This scarcity is what gives fees their market function. As the block subsidy continues to decline through successive halvings, transaction fees are expected to become a larger share of miner revenue. Understanding fees and the mempool is practical knowledge for anyone who sends bitcoin. It explains why confirmation times vary, why fees fluctuate, and how the network prioritizes transactions when demand exceeds capacity.