Mining

Bitcoin mining adds new blocks of transactions to the blockchain. Miners race to find a number that makes a block's hash small enough. The first miner to find one adds the block to the chain and earns the block reward. That reward is new bitcoin plus the fees in the block.

How does mining work?

Each node keeps a list of transactions it has seen but not yet confirmed. This list is the mempool. The mempool holds transactions that are still waiting, while the blockchain holds the settled ones. A miner picks transactions from its mempool, builds a new block from them, and starts work on that block.

The miner works on the block header, the short summary at the top of the block. The header holds the hash of the previous block, a fingerprint of all the transactions (the Merkle root), a timestamp, the target, and a number called the nonce. The blocks page describes each field.

A hash is a fixed-length fingerprint of some data. Change one character of the input, and the hash changes completely. Nothing lets a miner skip ahead to a winning result. So the miner hashes the header twice with SHA-256 and checks the result. If the hash is below the target, the miner has won. If not, the miner changes the nonce and tries again.

Most tries fail, so miners make a very large number of guesses. Specialized machines make those guesses at a huge rate. The more guesses a miner makes, the more often it wins. Each try is random, so even the fastest miner still depends partly on luck.

When a miner finds a winning hash, it shares the block with the network. Other nodes check the hash and the transactions. If everything is valid, they add the block to their own copy of the chain. The miner then starts the next block on top of the new one, and the race begins again. The Bitcoin developer guide covers these steps in more detail.

What is the target?

The target is the number a hash must fall below to count. A lower target means fewer hashes win, so a miner needs more guesses. The network sets the target through the difficulty. The difficulty changes every 2,016 blocks. That change keeps a new block arriving about every 10 minutes, however many miners are working.

Why does the network need mining?

Mining is how the network agrees on the next block. Imagine you pay for a coffee with a coin, then try to spend the same coin on a sandwich. Now the network has two payments that conflict. Transactions spread from node to node, a process called propagation, so nodes may have seen the two payments in different orders. Trying to spend the same coins twice is called a double spend.

Only one of the two payments can end up in the chain. The first valid block that includes one of them settles the matter. The other payment is no longer valid, because its coin is already spent, so nodes drop it.

No single miner can choose which transactions get in. To control the chain, a miner would need more hashing power than all the other miners combined. Mining also makes the record hard to change. Rewriting a block already in the chain would need that same hashing power, kept up for as long as the rewrite took.

A new block arrives about every 10 minutes. So a payment usually gets its first confirmation within about 10 minutes.

How much is the block reward?

The block reward has two parts. The first is new bitcoin, called the subsidy. The second is the fees paid by the transactions in the block. The miner claims both in the first transaction of the block, which is the coinbase transaction.

The subsidy started at 50 BTC in 2009. It halves every 210,000 blocks, which is about every four years. It has been 3.125 BTC since block 840,000 in April 2024. The next halving comes at block 1,050,000. Our halving page has the full schedule.

Can I mine bitcoin at home?

You can run mining hardware at home, but a home machine is very unlikely to find a block. Bitcoin Core has not included a miner since version 0.13.0, released in 2016. Today, mining runs on specialized machines that do nothing but hashing. Most miners therefore join a mining pool.

A pool combines the hashing power of many miners. When the pool finds a block, it splits the reward among its members, by the rules the pool sets.

Where can I see mining on Explore21?

Our mining page shows the pools that found recent blocks, the network's total hashrate, and when the next difficulty change is due. Open any block to see where its reward was paid. For example, block 100,000 shows its coinbase transaction and the output that paid the miner.

What should you read next?

Next, read the blockchain. It explains how blocks link together into the chain that every node keeps, and why that link matters.