SoloLuck

How a Bitcoin Transaction Really Works

← back to pool

SoloLuck Blog · 2026-07-01

There Are No Balances, Only Coins

When you look at a bank account, you see a single number: your balance. Bitcoin does not work this way. There is no ledger anywhere that stores a line saying this person owns 0.5 BTC. Instead, the network keeps track of individual chunks of bitcoin called unspent transaction outputs, or UTXOs.

A useful way to picture it is a wallet full of physical cash. You do not carry a balance in your pocket, you carry specific pieces: a twenty, a five, three ones. Your total is just the sum of those pieces. A Bitcoin wallet works the same way. What software shows you as a balance is really the wallet quietly adding up every UTXO your keys are able to spend. Follow a single payment from start to finish and this becomes surprisingly intuitive.

Inputs, Outputs, and Change

Every payment reuses coins you already received. A transaction has two sides:

Here is the part that surprises newcomers: a UTXO must be spent whole. Just as you cannot tear a twenty-dollar bill in half, you cannot spend only part of a UTXO. Say you hold a single 0.5 BTC output and want to send 0.2. Your wallet spends the entire 0.5, sends 0.2 to your friend, and sends the rest back to an address you control. That returned piece is called change, and it works exactly like the change a cashier hands back.

The gap between what goes in and what comes out is the transaction fee. It is not a separate charge you add on top; it is simply whatever is left over, total inputs minus total outputs. That leftover is claimed by the miner who eventually includes your transaction.

The Signature That Authorises It

What stops you from spending someone else's coins? A digital signature. Each UTXO is locked to an address, and only the matching private key can unlock it.

When you spend, your wallet uses that private key to sign the transaction. The signature proves two things at once: that you are authorised to spend those inputs, and that not a single detail, the amounts or the recipients, has been altered since you signed. Cleverly, it reveals your authority without revealing the key itself. Anyone can verify the signature; no one can work backwards from it to your private key.

This is why the phrase not your keys, not your coins matters. The private key is the ability to spend. Lose it and the coins are frozen forever; leak it and anyone can take them. Guarding that key is the whole game.

Broadcasting to the Network

A signed transaction is just a small bundle of data. To go anywhere, it has to reach the network, so your wallet broadcasts it to the Bitcoin nodes it is connected to.

Each node independently checks the transaction before passing it along. Are the signatures valid? Do the inputs actually exist and remain unspent? Do the numbers add up, with outputs never exceeding inputs? A transaction that fails any of these checks is quietly dropped and travels no further. One that passes is relayed to that node's peers, which relay it to theirs. Within seconds it has rippled out to thousands of nodes around the world. No single company approves it; the shared rules do.

The Mempool: The Waiting Room

A valid transaction is still not confirmed. It waits in each node's mempool (short for memory pool), the holding area for transactions that have been broadcast but not yet written into a block.

Miners build the next block out of mempool transactions, and because space in a block is limited, they tend to favour those offering a higher fee relative to the space they occupy, a figure known as the fee rate. This creates a simple, open market:

Nothing is lost while it waits; it is simply pending. A new block is found roughly every ten minutes on average, so there is always another chance coming soon.

Confirmations: When Is a Payment Final?

The moment a miner includes your transaction in a valid block, it earns its first confirmation. Your inputs are now marked spent, and your outputs exist as fresh UTXOs the recipient can spend in turn.

Each additional block mined on top adds another confirmation, burying your transaction a little deeper in the chain. Undoing it would mean re-mining that block and every block after it, a task that grows exponentially harder with each new block added. This is why Bitcoin's finality is called probabilistic: never a hard stamp of done, but a certainty that climbs quickly toward practically absolute.

By common convention, six confirmations, very roughly an hour, is treated as thoroughly settled for large amounts. For small everyday sums, one confirmation is often plenty. There is no central cleared button; you simply choose how much certainty a given payment deserves.

FAQ

What exactly is a UTXO?
A UTXO, or unspent transaction output, is a discrete chunk of bitcoin locked to an address, like a single bill in a wallet. Bitcoin has no account balances. What your wallet displays as a balance is just the sum of all the UTXOs your keys are able to spend.
Why did my wallet send bitcoin back to myself as change?
Because a UTXO must be spent whole. If you hold a 0.5 BTC output and want to send 0.2, the wallet spends the full 0.5, sends 0.2 to the recipient, and returns the rest to a change address you control, exactly like a cashier handing back change.
Why is my transaction stuck or unconfirmed?
An unconfirmed transaction is waiting in the mempool for a miner to include it in a block. Miners tend to favour higher fee rates, so a low fee during a busy period can leave it pending for a while. It is not lost; it is simply waiting its turn, and it can still be confirmed later.
How many confirmations does a Bitcoin transaction need?
There is no fixed rule. One confirmation means it is already in a block, and each later block makes reversal exponentially harder. Six confirmations, very roughly an hour, is a common convention for large amounts, while a single confirmation is often enough for small everyday payments.
Can a Bitcoin transaction be cancelled or reversed?
Once confirmed in a block it is effectively irreversible, and more so with every block added on top. While still unconfirmed in the mempool it has not truly settled, but you cannot simply cancel it. You can only wait, or in some cases replace it with a higher-fee version if your wallet supports that.

All blog posts · Mining guides · Start solo mining

Ready to take a ticket?

Paste your address and copy the config from /setup, watch the pool on /status, and check every claim on /verify. Mine to your own address — that is what makes it truly solo.

Get the setup config →

Not ready to point a miner yet? Run your gear through the odds calculator, or join Telegram for block & record alerts — no rig required.

More guides

Browse all guides →