Fees & the Mempool

The mempool, in plain English

Mempool stands for “memory pool,” the holding area where valid, unconfirmed transactions wait to be included in a block. Each node keeps its own mempool. Miners typically select transactions that pay the highest fee rates first.

Fee units

• sat/vB (satoshis per virtual byte): Bitcoin fee rates are quoted per virtual byte of transaction weight. Bigger transactions (more inputs, certain scripts) cost more.
• Total fee = fee rate × vbytes. Wallets usually estimate this for you.

Why fees rise and fall

• Congestion: When demand spikes (market volatility, popular inscriptions, airdrops, etc.), the mempool fills and higher fees are needed for priority.
• Block space is scarce: Blocks target ~10 minutes and have a maximum weight; only so many transactions fit.
• SegWit/Taproot efficiency: Using modern address types can reduce the vbytes required.

Getting confirmed faster (tools & tactics)

• Replace-By-Fee (RBF): If you underpaid, you can bump the fee by re-broadcasting a replacement transaction (must be marked as replaceable when created).
• Child-Pays-For-Parent (CPFP): Spend an output from the stuck transaction with a high-fee “child”; miners evaluate package fee rates.
• Batching: Combine multiple payments into one transaction to reduce total bytes per payment.
• Time preference: Choose a slower (cheaper) or faster (pricier) tier depending on urgency.

What makes a transaction “large”?

• Many inputs (e.g., sweeping a bunch of small UTXOs) increases size.
• Complex scripts or legacy address types can add weight versus SegWit/Taproot.

Best practices

• Prefer SegWit/Taproot addresses when possible for efficiency.
• Enable RBF in your wallet; confirm it’s on before sending.
• For big sends, simulate fees with your wallet’s estimator, or send a small test first.
• Consider batching and consolidating small UTXOs when fees are low.

Educational content only; not investment advice.

See also: Wallet types · Security hygiene · Back to “What is Bitcoin?”