# Difficulty

*See also: target*

## Contents

- 1 What is "difficulty"?
- 2 How often does the difficulty change?
- 3 What is the formula for difficulty?
- 4 How is difficulty stored in blocks?
- 5 What is the current difficulty?
- 6 What is the maximum difficulty?
- 7 Can the difficulty go down?
- 8 What is the minimum difficulty?
- 9 How soon might I expect to generate a block?

### What is "difficulty"?

Difficulty is a measure of how difficult it is to find a new block compared to the easiest it can ever be.

### How often does the difficulty change?

Every 2016 blocks.

### What is the formula for difficulty?

difficulty = maximum_target / current_target

(target is a 256 bit number)

### How is difficulty stored in blocks?

Difficulty is stored in blocks as a 4 byte integer, and the actual hexadecimal target is derived from it via a predefined formula. For example, if the difficulty in the block is 0x1b0404cb, the hexadecimal target is

0x0404cb << 2**(8*(0x1b - 3)) = 0x00000000000404CB000000000000000000000000000000000000000000000000

The highest possible target (difficulty 1) is defined as 0x1d00ffff, which gives us a hex target of

0x00ffff << 2**(8*(0x1d - 3)) = 0x00000000FFFF0000000000000000000000000000000000000000000000000000

So the difficulty at 0x1b0404cb is therefore:

0x00000000FFFF0000000000000000000000000000000000000000000000000000 / 0x00000000000404CB000000000000000000000000000000000000000000000000 = 16307.420938523983

### What is the current difficulty?

Current difficulty, as output by BitCoin's getDifficulty.

### What is the maximum difficulty?

The maximum difficulty is roughly: maximum_target / 1, which is a ridiculously huge number (about 2^224).

The actual maximum difficulty is when current_target=0, but we would not be able to calculate the difficulty if that happened. (fortunately it never will, so we're ok.)

### Can the difficulty go down?

Yes it can. See discussion in target.

### What is the minimum difficulty?

The minimum difficulty, when the target is at the maximum allowed value, is 1.

### How soon might I expect to generate a block?

(The eternal question.)

- Any one grinding of the hash stands the same chance of "winning" as any other. The numbers game is how many attempts your hardware can make per second.
- You need to know the difficulty (above) and your khash/sec rate (reported by the client).
- Mining Hardware Comparison has some stats that may help you predict what you could get.

- Visit a calculator or perform the maths yourself,
- Remember it's just probability! There are no guarantees you will win every N days.

*This article uses content from the old wiki. The list of contributors to the old page is available here.*