Fees and the mempool

After this lesson you can: read the mempool and choose a sensible fee.

After broadcast, a transaction sits in every node's mempool (memory pool) until mined. Each node's mempool is its own; they mostly agree but need not. Miners fill blocks from theirs, almost always by highest fee rate first.

Fee rate, not fee

Fees are measured in satoshis per virtual byte. Block space is the scarce thing, so you pay for the bytes your transaction occupies, not the value it moves. A simple single-input, two-output SegWit transaction is about 140 vbytes; at 10 sat/vB that is 1,400 sats.

Table of transaction sizes in virtual bytes for legacy, SegWit and Taproot payments and their fee at 10 sat/vB. — You pay for bytes, not value. A one-input, two-output payment by address type.

Why fees move

Block space is fixed at about 4 million weight units every ten minutes. When demand exceeds that (bull markets, token crazes, exchange outflows) fee rates rise; when it is quiet, 1 to 3 sat/vB confirms. Wallets estimate from the current mempool; a mempool explorer shows you the queue.

Horizontal bars showing an illustrative mempool: how much block space is waiting at each fee rate. — Illustrative mempool snapshot. Miners take the top bands first; a block clears about 4 million weight units.

Stuck transactions

Two ways to speed up a stuck transaction: replace it with a higher fee, or spend its output with a high-fee child. — Two rescues for a stuck payment.

Replace-by-fee (RBF): rebroadcast the same transaction with a higher fee; most wallets support it.
Child-pays-for-parent (CPFP): spend the stuck transaction's output with a high-fee child; miners take both.
Or wait: low-fee transactions confirm when demand drops, or expire from mempools after about two weeks and return to your balance.

The long run

Fees are meant to replace the block subsidy as miner income as it halves away. Whether fees will be enough is one of Bitcoin's genuine open questions; the monetary policy course takes it up.

MINI CHALLENGEPrice a transaction

A 140 vbyte transaction at 25 sat/vB pays how many sats in fees?

sats

CHECK YOURSELF

1/3 What do miners prioritise when filling a block?The oldest transactions, so nothing waits foreverHighest fee per byteThe largest amounts, because they matter most to the sender

2/3 What is RBF?A refund the network pays when a transaction is droppedReplacing an unconfirmed transaction with one paying a higher feeA type of address that lets the fee be paid by the receiver

3/3 How are fees measured?As a fixed percentage of the amount being sentIn dollars per transaction, set by the wallet companyIn satoshis per virtual byte, so the rate depends on transaction size rather than the amount sent

pick an answer for each question

Fee rate, not fee

Why fees move

Stuck transactions

The long run

SOURCES