How bitcoin works: ledger, keys, mining and the 21 million limit

This page explains the protocol rather than offering a live network, price or legal-status dashboard.

The problem bitcoin tries to solve

Digital things are easy to copy. If money were simply a file, what would stop someone from sending the same file to two people? In ordinary payments, a bank prevents that by keeping the official record of who owns what.

In 2008, a paper published under the name Satoshi Nakamoto proposed online payments sent “directly from one party to another without going through a financial institution”. The paper names the central difficulty, double-spending, and says the main benefits are lost if a trusted third party is still needed to prevent it. Bitcoin whitepaper.

A shared ledger instead of one bank’s records

Bitcoin’s answer is a ledger, a list of every payment ever made, that thousands of computers around the world each keep a copy of and check independently. New entries are added in batches called blocks, each linked to the one before it. That chain of blocks is the blockchain.

There are no coins inside the ledger and no coin files on anyone’s phone. The whitepaper defines an electronic coin as “a chain of digital signatures”. In practice, wallets calculate a balance from unspent transaction outputs (UTXOs). Spending an output must satisfy its conditions, commonly a signature, sometimes multiple signatures or other script conditions.

What you actually own: a private key

A private key is a very large secret number. It lets its holder create signatures that move the bitcoin assigned to the matching address. Nodes can verify signatures without learning the private key. Security depends on sound cryptography, wallet software and keeping secrets protected.

So owning bitcoin, in a technical sense, means controlling a key. Whoever has the key controls the coins. Many wallets provide recovery words representing a seed from which multiple keys are derived. They are not simply one private key written as words. Recovery depends on the wallet scheme, any additional passphrase and a correct backup.

How a payment moves

1. Your wallet selects unspent outputs and creates new outputs for the recipient and, usually, change.
2. Your wallet signs it with your private key.
3. The message is broadcast to the network and waits, together with other pending payments, to be included in a block.
4. Once a block containing it is added to the chain, the payment is recorded. Each further block on top makes it harder to reverse.

For a simplified example, inputs of 0.4 and 0.3 BTC total 0.7 BTC. Outputs of 0.5 BTC to a recipient and 0.199 BTC as change leave a 0.001 BTC fee. That fee is an arithmetic assumption, not a recommended fee.

Fees depend mainly on how much data a payment takes up and how busy the network is, not on the amount sent.

Mining and difficulty

Miners collect pending payments into a block. To add it to the chain, they must find a number that gives the block’s digital fingerprint (its hash) a required form. There is no shortcut: miners guess, check and guess again.

The winning miner adds the block, and every other computer checks it before accepting it. If competing versions appear, the network follows the chain with the most accumulated work; the whitepaper calls it the “longest chain”.

Every 2,016 blocks, the puzzle’s difficulty adjusts automatically so that blocks keep arriving about every ten minutes on average, whether more or fewer machines are mining. Bitcoin Core consensus parameters.

Miners do this because the first entry in each block pays the winner newly created bitcoin plus the fees from the payments inside.

Supply schedule and halvings

The block subsidy creates new bitcoin; transaction fees are existing bitcoin transferred to the miner. The subsidy, and it follows a fixed schedule written into the software:

• The subsidy started at 50 bitcoin per block.
• Every 210,000 blocks, roughly every four years, it halves.
• The most recent halving was at block 840,000 on April 20, 2024, cutting the subsidy from 6.25 to 3.125 bitcoin. Block 840,000.
• The next halving is due at block 1,050,000. At about ten minutes per block, that is around April 2028; the exact date depends on actual block times.

Because each halving cuts new issuance in half, the total converges just under 21 million bitcoin, with the last fractions expected around the year 2140. The 21 million figure comes from the software rules, not from the whitepaper.

The halving is a rule about issuance. It says nothing reliable about future prices.

Lost coins

Issued supply differs from spendable supply. If the required keys and all usable backups are lost, the network has no password-reset service. An inactive address alone does not prove loss: someone may be deliberately holding the coins. Estimates of lost supply are therefore uncertain.

Energy use

Proof-of-work uses electricity to perform repeated hashing. Energy use and emissions depend on mining equipment, utilisation and electricity sources. Neither a rising price nor a fixed supply cap tells you the environmental footprint. The protocol’s security mechanism has a real resource cost.

Custody: who holds the key?

For ordinary users, this is the most important practical question.

Holding the key yourself (self-custody). You do not depend on a custodian to approve a withdrawal. That does not make coins immune to theft, transaction censorship, legal seizure or loss of access. There is no central recovery service. If you lose the key or recovery words, or are tricked into revealing them, the coins are gone. Self-custody also carries theft risk: Chainalysis counted about 158,000 personal-wallet theft incidents across crypto assets in 2025, affecting at least 80,000 individual victims. Chainalysis 2025 theft report.

Leaving the key with a platform. This is convenient, but you usually hold a claim against that company, not the coins themselves. If the platform fails, is hacked or commits fraud, you may become a creditor in its insolvency. Do not assume bank-deposit protection applies because an app looks like a bank account. SEC Investor.gov (US): crypto assets.

These risks are not theoretical:

• In November 2022, one of the world’s largest crypto exchanges collapsed. Its founder was later sentenced in the US to 25 years in prison; the US Attorney said he had stolen more than $8 billion of customers’ money. US Department of Justice, March 2024.
• More than $3.4 billion in crypto was stolen worldwide in 2025, about $1.5 billion of it in a single hack of one exchange in February 2025. Chainalysis 2025 theft report.

Common scam patterns include promises of guaranteed or fixed returns, requests for your key or recovery words, and demands for a “fee” or “tax” to unlock or withdraw coins. A legitimate service never needs your recovery words.

Why the price swings so much

A bitcoin pays no native interest, dividends or rent. Market prices respond to demand, available supply, liquidity and news. Price swings can be large; scarce supply alone does not guarantee purchasing power or positive returns.

As a hypothetical illustration, 1,000 units falling 60% become 400. Recovering from 400 to 1,000 requires a 150% gain, not 60%. This arithmetic is not a price forecast. Read crypto risks: questions beyond the price chart for custody, liquidity and loss questions.

How the rules differ by region

Rules for exchanges, custody, products, advertising and taxes vary by country and change over time. Check the relevant regulator and the exact legal entity serving you before relying on a platform.

For a historical example, on January 10, 2024, the US SEC approved the listing and trading of specified spot bitcoin exchange-traded products. Its statement explicitly distinguished that decision from endorsing bitcoin. SEC statement.

Regulating a business around bitcoin does not remove the asset’s volatility or guarantee recovery after theft. Product approval also does not mean the underlying asset is suitable for you.

Four questions to ask before any crypto decision

1. What exactly would I own? A key that controls coins, or a claim on a company?
2. Who holds the key, and what happens if they disappear? Read the terms on custody, insolvency and withdrawals.
3. Could I lose all of it without changing my life? If not, the amount is too large for an asset that has repeatedly fallen by more than half.
4. Is anyone promising returns or asking for my recovery words? If so, treat it as a scam.

Related reading

• Crypto risks: questions beyond the price chart covers custody, liquidity and fraud questions in more depth.
• An asset without cash flows sits on the speculative side of the line drawn in investing vs. gambling: a 7-question test.
• Before taking any high-risk position, check your safety net with the recession stress test and how to start an emergency fund.
• Deposit insurance explains how to check which protection applies to a balance, and why a claim on an intermediary is not the same as an insured bank deposit.