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3.1 Introduction

Instead, Bitcoin is made up of individual pieces of value called UTXOs. Unspent Transaction Outputs. Understanding UTXOs is essential for properly designing, building, and reasoning about Bitcoin wallets.

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3.2 What is a UTXO?

A UTXO is a piece of bitcoin that:
  • Has been sent to a Bitcoin address.
  • Has not yet been spent.
  • Is ready to be used as input to a future transaction.
Every time someone sends bitcoin, they are spending old UTXOs and creating new UTXOs. The blockchain is essentially a database of all unspent outputs. Key insight: Bitcoin is not a balance ledger. Bitcoin is a collection of spendable digital coins (UTXOs) linked to addresses.

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3.3 Example: UTXOs in action

Suppose:
  • Ali receives 0.5 BTC from Dapo.
  • Later, Ali receives 1.0 BTC from Kwame.
Ali’s wallet now controls:
  • UTXO1: 0.5 BTC
  • UTXO2: 1.0 BTC
His “balance” (what the wallet app shows) is: 0.5 BTC + 1.0 BTC = 1.5 BTC But behind the scenes, it is two separate pieces. If Ali wants to send 0.8 BTC to Dapo:
  • His wallet must choose UTXOs to spend.
  • Likely, the wallet will spend UTXO2 (1.0 BTC) fully.
  • Create an output sending 0.8 BTC to Dapo.
  • Return the leftover 0.2 BTC back to Ali as change.
Thus, a new UTXO is created for Ali for the change.

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3.4 Visual diagram: UTXO flow

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3.5 Important properties of UTXOs

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3.6 Wallet responsibilities with UTXOs

A Bitcoin wallet must:
  • Keep track of all UTXOs controlled by the user’s private keys.
  • Select appropriate UTXOs when building a transaction (smart coin selection).
  • Handle change outputs carefully to maintain privacy and efficiency.
  • Consolidate small UTXOs when network fees are low.
  • Warn users or sweep dust when necessary.
Even a simple “send” button click involves complex UTXO selection behind the scenes.

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3.7 UTXO selection strategies

Wallets must often choose which UTXOs to use when sending bitcoin. Different strategies impact:
  • Transaction fees
  • Privacy
  • UX performance
Common strategies: Well-designed wallets adapt strategies based on transaction size, urgency, and user preferences.

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3.8 Dust and wallet bloat

Dust = very small UTXOs that cost more to spend than they are worth. Example: A UTXO of 200 satoshis. If it costs 400 satoshis in fees to spend, it is dust. Wallets must:
  • Avoid creating dust outputs.
  • Consolidate dust during periods of low network fees.
  • Warn users when transactions would produce dust.
Wallet bloat: Accumulating hundreds or thousands of tiny UTXOs leads to:
  • Expensive transactions.
  • Slower wallet performance.
  • Difficulty moving funds quickly.

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3.9 PM reflection points

If you are managing a Bitcoin wallet product:
  • Always remember that users’ “balances” are not simple numbers. They are collections of UTXOs.
  • Wallet apps must hide UTXO complexity, but handle it carefully behind the scenes.
  • Smart UTXO management improves:
    • Fee efficiency
    • Privacy
    • Long-term wallet usability
  • Backup flows must protect all keys necessary to spend future UTXOs.
  • Change addresses must be rotated automatically to protect privacy.
Core idea: Every Bitcoin transaction a user makes is a negotiation with the past. Selecting UTXOs to build the future.

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3.10 Summary of module 3

  • Bitcoin’s data model is a system of UTXOs, not account balances.
  • UTXOs are indivisible chunks of bitcoin locked to addresses.
  • Transactions consume old UTXOs and create new UTXOs.
  • Wallets are responsible for tracking, selecting, and managing UTXOs intelligently.
  • Poor UTXO management leads to high fees, privacy loss, and poor user experience.