Transactions
Inputs, outputs, UTXOs, fees and confirmations: how a Bitcoin payment is actually built.
What is a Bitcoin transaction?
A Bitcoin transaction spends existing coins (inputs) and locks new ones to their next owners (outputs). See how it works with a real example.
UTXOs: how Bitcoin balances really work
Bitcoin has no account balances. Your balance is the total of unspent transaction outputs (UTXOs) your keys can unlock. Learn how UTXOs work.
Transaction inputs
An input points to an earlier output by txid and index, and supplies the signature that unlocks it. Learn what an input contains and why it has no amount.
Transaction outputs
Every output is an amount in sats plus a locking script that says who can spend it next. Learn how outputs, addresses and output numbers fit together.
Change outputs
When you spend a coin worth more than the payment, the rest returns to you as a change output. Learn why change exists and why it can matter for privacy.
How Bitcoin transaction fees work
Bitcoin fees are inputs minus outputs, priced by size in sat/vB, not by amount sent. Learn what sets fees, how to pay less and how to fix a stuck payment.
Fee rate: sats per vbyte
Fee rate is the fee divided by virtual size, in sats per vbyte (sat/vB). It is the number miners sort by. Learn how to calculate and choose one.
Transaction size, weight and vbytes
Since SegWit, transactions are measured in weight units and vbytes, with witness data discounted. Learn how size is counted and why it sets your fee.
Transaction IDs (txid)
A txid is the double SHA-256 hash of a transaction, shown in reverse byte order. Learn how it is made, where to find it and how to look one up.
What is a Bitcoin confirmation?
A confirmation means your transaction is in a block, and each new block adds another. Learn how many you need, how long they take and why they matter.
Replace-by-fee (RBF)
Replace-by-fee lets you resend an unconfirmed Bitcoin transaction with a higher fee so it confirms faster. Learn the rules and how wallets do it.
Child pays for parent (CPFP)
CPFP speeds up a stuck Bitcoin transaction by spending its output in a new high-fee child. Miners take both together. Learn when and how to use it.
The coinbase transaction
The first transaction in every block, created by the miner, pays out the new-coin subsidy plus all fees. Learn its special rules and famous messages.
Locktime and sequence
Locktime stops a transaction confirming before a set block or time; sequence numbers enable RBF and relative timelocks. Learn how both work.
Dust and UTXO consolidation
Dust is an output worth less than it costs to spend. Learn the dust limit, when to consolidate small UTXOs to save fees, and the privacy trade-off.
Transaction batching
Batching pays many recipients in one Bitcoin transaction, sharing fixed costs and saving block space. See how much it saves and who uses it.
Anatomy of a raw transaction
Every Bitcoin transaction is a string of bytes: version, inputs, outputs, witness and locktime. Learn the layout and decode raw transaction hex.
PSBT v0 and v2: format versions, not network forks
Compare the original PSBT format with version 2, including input/output construction, modification flags and compatibility between signing tools.
CompactSize and VARINT: two different encodings
Decode Bitcoin CompactSize lengths with worked byte examples, distinguish Core’s internal VARINT format, and avoid unsafe parser assumptions.