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Unlocking the Mystery: How Bitcoin's Coin Selection Algorithm Works

Bitcoin's Coin Selection Algorithm


Bitcoin's Coin Selection Algorithm is a crucial yet often overlooked aspect of how transactions are processed. It determines which unspent transaction outputs (UTXOs) are used as inputs when you make a transaction. Understanding this algorithm can help you appreciate the efficiency and genius behind Bitcoin's design. This article aims to demystify the algorithm, breaking it down into simple terms for beginners.

What is a UTXO?

Before diving into the algorithm, it's essential to understand what a UTXO is. UTXO stands for Unspent Transaction Output. Think of it as the 'change' you have in your Bitcoin wallet. Each time you make a transaction, you use these UTXOs, and new ones are generated as 'change.'

The Algorithm at a Glance

The Coin Selection Algorithm aims to optimize the use of UTXOs in a transaction. It does so by following a series of steps to find the most efficient combination of UTXOs to match the transaction amount, known as the "Target."

How Does It Work?

  1. Exact Match: If any of your UTXOs matches the Target amount, it will be used.

  2. Sum of Smaller UTXOs: If the sum of all your UTXOs smaller than the Target matches the Target, they will be used.

  3. Smallest Greater UTXO: If the sum of all your UTXOs smaller than the Target doesn't surpass the target, the smallest UTXO greater than your Target will be used.

  4. Random Combination: Else, Bitcoin Core does 1000 rounds of randomly combining UTXOs until their sum is greater than or equal to the Target. If an exact match is found, it stops early and uses that.

  5. Final Selection: Otherwise, it settles for the minimum of the smallest UTXO greater than the Target or the smallest combination of UTXOs discovered in step 4.

Real-World Example

Let's say Alice has four UTXOs: 0.1BTC, 0.3BTC, 0.5BTC, and 1BTC. She wants to send 0.4BTC to Bob. The algorithm will find that the sum of all UTXOs smaller than the target (0.1BTC + 0.3BTC = 0.4BTC) matches the Target. Therefore, both 0.1BTC and 0.3BTC UTXOs will be used as inputs for the transaction.

Why Is This Important?

Understanding the Coin Selection Algorithm can help you appreciate how Bitcoin optimizes transactions, minimizes fees, and enhances privacy. It also sheds light on why sometimes transactions take longer to confirm, especially if the algorithm has to go through multiple rounds to find the best UTXOs.


Bitcoin's Coin Selection Algorithm is a masterpiece of efficiency and optimization. While it may seem complex at first glance, understanding its basics can provide valuable insights into how Bitcoin transactions work. So the next time you make a Bitcoin transaction, you'll know there's a lot more going on behind the scenes than you thought.

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