Learn Bitcoin

Three kinds of statement

As you read, notice which kind of claim you are looking at. Protocol rules are fixed and enforced by every node — they do not change because someone wants them to. Live conditions describe what the network is doing right now and will be different tomorrow. Estimates are researchers’ inferences and are contested. Keeping these apart is most of what it takes to read Bitcoin data well.

Bitcoin is a shared ledger that nobody administers. Thousands of independent computers each keep a full copy, and each one checks every rule for itself before accepting anything new. There is no server to ask, no company that can freeze an entry, and no administrator who can create additional units.

What makes that work is that the rules are cheap to verify and expensive to break. Anyone can check that a transaction is validly signed in microseconds. Rewriting history, on the other hand, would mean redoing an enormous amount of computational work — currently — blocks of it — faster than the rest of the network extends it.

Transactions are gathered into blocks, and each block commits to the one before it. That chain is what makes ordering unambiguous: to change an old transaction you would have to rebuild every block since.

Blocks arrive roughly every ten minutes — currently averaging —. The interval is a deliberate compromise. Shorter would mean faster confirmation but more competing blocks found simultaneously, wasting work and weakening the ordering guarantee. Longer would mean waiting even more for a payment to settle.

See recent blocks

To add a block, a miner must find a number that makes the block’s hash fall below a target. There is no shortcut — you guess, trillions of times a second, until one works. That expenditure is what a block’s security actually is: a receipt for energy spent.

Because hardware keeps improving, the target moves. Every 2,016 blocks the network measures how long they took and adjusts difficulty so the next 2,016 land closer to two weeks. This is why blocks keep arriving at the same pace whether the network is secured by one computer or by millions.

Watch the current retarget · See the hashrate behind it

Each block pays its miner a subsidy of newly created bitcoin — currently —. Every 210,000 blocks that subsidy halves. It has halved four times, and after 33 halvings the integer arithmetic reaches zero and issuance stops for good.

That schedule is why the total is capped at 21 million. Around — of it already exists, and — blocks remain until the next halving. Nobody administers this: a block claiming more than its permitted subsidy is rejected by every node, automatically.

See issuance against the cap · The halving countdown

A broadcast transaction does not go into a block immediately. It waits in the mempool — currently holding about — transactions — until a miner includes it.

Block space is limited, so when more transactions want in than fit, they compete. Miners take the highest fee rates first, which is why fees are quoted per virtual byte rather than as a percentage: you are paying for space, not for value. Sending a very large amount costs the same as sending a small one if the transactions are the same size. The current next-block rate is —.

See the current queue · Price a transaction

There are no accounts in Bitcoin. What exists is a set of unspent outputs — discrete chunks of bitcoin, each locked to a condition. Spending means consuming whole outputs and creating new ones, much like paying cash: hand over a note larger than the price and receive change.

Your wallet’s “balance” is just the sum of the outputs it can unlock. This is also why fees vary so much between wallets: each output you consume adds bytes, so a wallet holding many small outputs pays noticeably more to spend them than one holding a few large ones.

An output is locked to a condition that normally requires a signature from a specific private key. Whoever can produce that signature can spend it. There is no recovery process, no identity check, and nobody who can override it.

That is the whole of what “not your keys, not your coins” means. If a third party holds the key, you hold a claim against that party rather than bitcoin itself — a claim that depends on their solvency and their honesty. Neither arrangement is universally correct, but they are genuinely different things and are worth not confusing.

Every Bitcoin transaction must be stored by every node forever, which puts a hard ceiling on throughput. Lightning works around it: two parties lock funds in a shared on-chain output and then exchange signed updates between themselves, settling to the chain only when they finish. Payments route across chains of these channels, so you can pay someone you have no direct channel with.

See public Lightning statistics