The UTXO Model

Banks track balances. Your account has a number — say $1,000 — and transactions increase or decrease it. Simple, familiar, intuitive.

Bitcoin doesn’t work that way.

Bitcoin tracks ownership through UTXOs — Unspent Transaction Outputs. Rather than a balance, your wallet is a collection of individual coin parcels — each one the output of a previous transaction that hasn’t been spent yet.

Here’s a simple example. Someone sends you 0.1 Bitcoin. That creates a UTXO: 0.1 Bitcoin, spendable only by the private key associated with your address. A week later, someone sends you 0.05 Bitcoin. That’s a second UTXO. Your wallet shows a balance of 0.15 Bitcoin — but underneath, it holds two separate coin parcels.

When you spend Bitcoin, you spend UTXOs — whole ones. If you want to send 0.08 Bitcoin, you might spend the entire 0.1 Bitcoin UTXO, send 0.08 to the recipient, and send the remaining 0.02 back to yourself as change — creating a new UTXO you control.

This is why Bitcoin transactions produce change addresses. Not because the technology is clunky — but because the UTXO model requires spending whole coin parcels and returning the remainder.

Why does this design exist? Privacy and verifiability. Each UTXO has a clear, traceable history. The network can verify at any point that a UTXO hasn’t been double-spent — without needing to track account balances. Every unspent output in existence can be compiled into the UTXO set, which represents the complete ownership state of all Bitcoin at any moment.

It’s a different model from banking. Once understood, it’s an elegant one.

Tomorrow: hash rate — what it measures and what it tells you about Bitcoin’s security.

— The Daily Bit