How a Bitcoin transaction really works (UTXOs)

Your "balance" is really a pile of digital banknotes.

Builder · 10 min · Updated 2026-11-08

💡 Paying with banknotes and getting change

You want to buy a ₹300 shirt but only have a ₹500 note. You can't tear the note — you hand over the whole ₹500 and get ₹200 back. Bitcoin works the same way: your wallet holds separate "notes" called UTXOs (unspent transaction outputs). To pay, you spend whole notes and send the leftover back to yourself as change — minus a small tip for the miner.

Balances are an illusion — outputs are real

The Bitcoin ledger doesn't store "Alice: 0.05 BTC". It stores outputs: chunks of bitcoin locked to a key. Your wallet adds up the unspent ones (UTXOs) and shows you a balance.

Anatomy of a transaction

• Inputs — references to UTXOs you are spending, each proven with your signature.
• Outputs — new UTXOs: one for the recipient, usually one "change" output back to you.
• Fee — not written as a field! It is simply sum(inputs) − sum(outputs), and the miner keeps it.

Fees depend on size, not amount

Miners prioritise transactions paying more satoshis per virtual byte (sat/vB). Sending $10 or $10 million costs the same fee if the transaction is the same size. Using many small UTXOs makes a transaction bigger — and costlier.

From mempool to confirmation

1. Your signed transaction is broadcast and waits in nodes' mempool (waiting room).
2. A miner picks the highest-paying transactions that fit in a block.
3. Once in a block: 1 confirmation. Each new block on top adds another.

🧮 Practical example: Alice pays Bob 0.01 BTC

Key takeaways

• Wallet balance = sum of your UTXOs.
• Inputs are spent whole; leftover returns as change.
• Fee = inputs − outputs, priced in sat/vB.
• Confirmations accumulate as blocks stack on top.

Education only — not financial advice.