UTXO

If you've ever looked at a Bitcoin transaction in a block explorer, you may have noticed it doesn't look like a simple "send money from A to B" bank transfer. Instead, it shows inputs and outputs. That's because Bitcoin tracks money using UTXOs, a clever system that works a lot like the cash in your wallet.

What Does UTXO Mean?

UTXO stands for unspent transaction output. Every Bitcoin transaction takes some existing outputs as inputs and creates new outputs. An output that hasn't been spent yet is a UTXO. Your wallet balance isn't stored as a single number on the blockchain. It's the sum of all the UTXOs that your keys can unlock.

Think of paper money. Say you have a $20 bill and a $5 bill in your wallet, so your balance is $25. If you buy something for $12, you hand over the $20 bill and get $8 back in change. You can't tear the bill in half. Bitcoin works the same way: each UTXO is like a bill of a specific size, and spending one often creates change.

How UTXOs Work

The UTXO model is the foundation of how Bitcoin records ownership. Here's what happens step by step when you send a payment.

Inputs

When you send bitcoin, your wallet picks one or more of your UTXOs to use as inputs. Together, they must cover the amount you're sending plus the transaction fee. Each input has to be unlocked with a valid signature from the matching private key, proving you're allowed to spend it.

Outputs

The transaction then creates new outputs. One goes to the person you're paying. If your inputs add up to more than you need, another output sends the leftover amount back to you, usually to a new "change address" in your own wallet. These new outputs become fresh UTXOs that can be spent later.

Fees

Bitcoin transactions don't list the fee as a separate payment. Instead, the fee is the difference between the total inputs and the total outputs. Whatever isn't assigned to an output is collected by the miner who includes the transaction in a block.

All or Nothing

A UTXO must always be spent in full. Just like that $20 bill, you can't spend part of it and leave the rest in place. That's why change outputs are so common.

A Simple Example

Imagine your wallet holds two UTXOs, one worth 0.3 BTC and one worth 0.5 BTC. You want to send 0.6 BTC to a friend. Your wallet uses both UTXOs as inputs, totaling 0.8 BTC. It creates one output of 0.6 BTC for your friend and one change output of about 0.1999 BTC back to you. The remaining 0.0001 BTC is the fee.

After the transaction confirms, your original two UTXOs are marked as spent, your friend has a new 0.6 BTC UTXO, and you have a new change UTXO. If you looked up the transaction on a block explorer, you'd see exactly this: two inputs on one side, two outputs on the other, and the fee calculated from the difference.

Every node on the network keeps a list of all current UTXOs, often called the UTXO set. When a new transaction arrives, nodes check that each input is in this set and hasn't already been spent. That simple check is a big part of how Bitcoin prevents anyone from spending the same coins twice.

UTXO Model vs Account Model

Not every blockchain uses UTXOs. Ethereum uses an account model, which works more like a bank account with a balance that goes up and down. Each model has its own strengths:

• Simplicity: The account model is easier to understand and makes smart contracts more straightforward to build.
• Verification: With UTXOs, each output can be checked independently, which makes it easier to verify transactions in parallel.
• Double-spend protection: A UTXO can be spent only once, so the network simply checks whether it still exists.
• Privacy: Using new change addresses can make it harder to link all your funds together, though blockchain analysis can often connect them.

Bitcoin, Litecoin, Bitcoin Cash, and Dogecoin use the UTXO model. Cardano uses an extended version that supports more complex smart contracts.

Why UTXOs Matter to You

You don't need to manage UTXOs manually to use Bitcoin, because your wallet handles them automatically. Still, understanding them explains a few things you may notice.

Fees Depend on Size, Not Amount

Bitcoin fees are based on how much data a transaction takes up, not how much money you send. A transaction that uses many small UTXOs as inputs is larger and costs more than one using a single big UTXO, even if the amounts are the same.

Dust

Very small UTXOs, often called dust, can cost more in fees to spend than they're worth. If you receive lots of tiny payments, consolidating them when network fees are low can save money later.

Change Addresses

If you see your wallet using a new address after you send funds, that's usually where your change went. It's still your money, protected by the same seed phrase.

Useful Links

• What is the UTXO model?
• What is Bitcoin?
• What are blockchain explorers?
• What is a mempool?