Understanding UTXO: A Comprehensive Guide

Demystifying UTXO: What it is and how it works

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Written by cavpatrol
Updated over a week ago

If you've ever delved into the world of cryptocurrencies or blockchain technology, you've probably come across the term "UTXO." But what exactly is a UTXO, and why is it crucial to understanding how cryptocurrencies like Bitcoin function? In this guide, we'll break down the concept of UTXO in simple terms, providing you with a clear understanding of its significance, particularly in reference to the emerging Bitcoin Ordinals ecosystem.

What is UTXO?

UTXO stands for Unspent Transaction Output. It is a fundamental concept in blockchain technology and plays a pivotal role in how cryptocurrencies work, especially Bitcoin.

To comprehend UTXO, let's first understand how transactions work within a blockchain:

Inputs and outputs

In a cryptocurrency transaction, there are inputs and outputs. Inputs are the sources of funds (previously received transactions), while outputs are the destinations where the funds are going.

Spent vs. unspent

Each output from a previous transaction can be in one of two states - spent or unspent. If it hasn't been used in any subsequent transactions, it's considered unspent. Conversely, once an output has been used in a transaction, it becomes spent.

UTXOs defined

UTXOs are the unspent outputs of previous transactions. They represent the cryptocurrency that a user possesses and can use in future transactions.

Transactions create UTXOs

When you make a transaction, you are essentially spending some of your UTXOs and creating new ones for the recipients. The leftover value from your previous UTXOs becomes new UTXOs for your own use.


Imagine you have three UTXOs:

  • UTXO1: 0.2 BTC

  • UTXO2: 0.3 BTC

  • UTXO3: 0.2 BTC

You want to send 0.4 BTC to a recipient. To make this transaction, you use UTXO1 (0.2 BTC) and a portion of UTXO2 (0.2 BTC) to create a new UTXO for the recipient, totaling 0.4 BTC.

You also create a change UTXO for yourself from the remaining portion of UTXO2. Since you used 0.2 BTC from UTXO2 for the recipient, you have 0.1 BTC left from UTXO2 as change.

  • UTXO1 is spent (0.2 BTC)

  • UTXO2 is partially spent (0.2 BTC sent to the recipient)

  • UTXO3 remains unspent (0.2 BTC)

  • UTXO4 did not exist initially but is created with 0.4 BTC sent to the recipient.

  • UTXO5 did not exist initially but is created with 0.1 BTC as change from UTXO2.

Here is a detailed breakdown of the inputs and outputs:





Your Address


0.2 BTC

Your Address


0.3 BTC

Your Address


0.2 BTC



UTXO (New)




0.4 BTC

Your Address

UTXO5 (Change)

0.1 BTC

Unspent UTXO:




Your Address


0.2 BTC

Order of UTXO Spending in Bitcoin Transactions

In the Bitcoin UTXO model, the order in which UTXOs (Unspent Transaction Outputs) are spent follows a "First In, First Out" (FIFO) approach by default. This means that the UTXOs received earliest are spent first when initiating a transaction. When a user creates a Bitcoin transaction, it selects UTXOs to cover the transaction amount, including fees. Any excess value from these UTXOs is returned as a new UTXO, which also adheres to the FIFO rule.

It is important to note that some advanced wallet software allows users to manually select which UTXOs to spend, offering more flexibility in determining the order of UTXO spending. This is particularly important when considering Ordinal theory.

Ordinal theory and UTXOs

Ordinal Theory is a method for uniquely labeling every satoshi, the smallest unit of the Bitcoin currency, with a distinct number. With Bitcoin having a total supply of 21 million (or 2.1 quadrillion satoshis), these numbers are assigned in the order they are mined. When a new block is mined, new satoshis with unique numbers are created in the coinbase transaction. These satoshis exist solely within UTXOs.

Ordinal Theory effectively transforms fungible token satoshis into non-fungible tokens, providing a basis for Digital Artifacts on Bitcoin.

Why UTXOs matter

Understanding UTXOs is crucial for several reasons:

  • Security: UTXOs provide a high level of security because they ensure that each transaction only spends the specific UTXOs it references. This prevents unauthorized access to your entire balance.

  • Transaction Tracking: UTXOs make it easy to track the flow of cryptocurrency within the blockchain. This transparency enhances security and helps prevent double-spending.

  • Privacy: UTXOs offer some degree of privacy since they don't reveal your entire wallet balance in each transaction. Instead, only the UTXOs used in that transaction are disclosed.

In summary, a UTXO, or Unspent Transaction Output, is a foundational concept in blockchain technology, particularly in cryptocurrencies like Bitcoin. It represents the unspent funds from previous transactions, which users can utilize in new transactions. Understanding UTXOs is essential for secure, transparent, and efficient blockchain operations. Now that you have a grasp of this concept, you're better equipped to navigate the fascinating world of cryptocurrencies and blockchain technology.

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