Bitcoin Difficulty

Difficulty Epoch # Progress: %

Bitcoin Difficulty Chart (Historical)

Bitcoin Difficulty FAQ

What is Bitcoin difficulty?

Bitcoin difficulty determines how hard it is to mine a new block. It ensures that blocks are found roughly every 10 minutes, no matter how much mining power is on the network.

Mining is like a lottery where miners repeatedly guess numbers (hashes) to find one that meets the required target. The difficulty level adjusts every 2,016 blocks (about two weeks) to keep mining steady. If blocks are found too quickly, difficulty increases. If they take too long, it decreases.

Each difficulty setting lasts for one adjustment period, called a "difficulty epoch." This system keeps Bitcoin’s issuance rate predictable, regardless of how many miners participate.

How is Bitcoin Difficulty Calculated?

Bitcoin difficulty is recalculated every 2,016 blocks. If blocks were mined too quickly, difficulty increases. If they took longer than 10 minutes on average, difficulty decreases. This adjustment keeps block times stable.

The formula for difficulty is:

Difficulty = Difficulty Target / Current Target

• Difficulty Target is the highest possible target a valid block hash can reach.
• Current Target is a hexadecimal number set in the block header.

In simple terms, difficulty represents how many hashes miners must try, on average, to find a valid block.

Why Is Bitcoin's Target Block Time 10 Minutes?

Bitcoin aims for a 10-minute block time using its difficulty adjustment algorithm. But why 10 minutes—not 1 minute or an hour?

Satoshi Nakamoto never gave a clear explanation, but there’s a practical reason behind this choice.

When a block is mined, it takes time for the news to spread across the network. During that delay, other miners might still be working on the previous block, unaware a new one exists. If two miners find valid blocks at nearly the same time, only one will be accepted. The other becomes an “orphan” block, and all the work behind it is wasted.

A longer interval between blocks gives the network more time to sync, reducing the chances of wasted effort. For example, if it takes around 1 minute to hear about a new block and blocks are found every 10 minutes, only about 10% of mining work is potentially wasted. If blocks were mined every minute, that inefficiency would spike.

So, a 10-minute target strikes a balance: frequent enough for steady transaction processing, but long enough to minimize wasted work.

What Is a Difficulty Epoch?

A difficulty epoch is a period of 2,016 Bitcoin blocks. With a target block time of 10 minutes, one epoch lasts roughly two weeks.

At the start of each epoch, the Bitcoin network adjusts mining difficulty. It does this by measuring how long it took to mine the previous 2,016 blocks.

The expected time is 20,160 minutes (2,016 × 10 minutes). If the actual time was shorter, blocks were mined too quickly, so the difficulty goes up. If it took longer, the difficulty goes down.

This regular adjustment helps keep Bitcoin's block production steady, regardless of how much mining power is on the network.

A Quirk in Bitcoin’s Difficulty Adjustment

There’s a known bug in Bitcoin’s source code that affects how difficulty is recalculated.

When adjusting difficulty, the network only considers the time it took to mine the last 2,015 blocks—not the full 2,016. As a result, the first block in each difficulty epoch is ignored in the calculation.

This means that even if the first block took an unusually long time—say, a whole year—it would have no impact on the next difficulty adjustment. Strange but true.