# How to Build Your Own Blockchain Part 4.1 — Bitcoin Proof of Work Difficulty Explained

If you’re wondering why this is part 4.1 instead of part 4, and why I’m not talking about continuing to build the local jbc, it’s because explaining Bitcoin’s Proof of Work difficulty at a somewhat lower level takes a lot of space. So unlike what this title says, this post in part 4 is not how to build a blockchain. It’s about how an existing blockchain is built.

My main goal of the part 4 post was to have one section on the Bitcoin PoW, the next on Ethereum’s PoW, and finally talk about how jbc is going to run and validate proof or work. After writing all of part 1 to explain how Bitcoin’s PoW difficulty, it wasn’t going to fit in a single section. People, me included, tend get bored in the middle reading a long post and don’t finish.

So part 4.1 will be going through Bitcoin’s PoW difficulty calculations. Part 4.2 will be going through Ethereum’s PoW calculations. And then part 4.3 will be me deciding how I want the jbc PoW to be as well as doing time calculations to see how long the mining will take.

The sections of this post are:

1. Calculate Target from Bits
2. Determining if a Hash is less than the Target
3. Calculating Difficulty
4. How and when block difficulty is updated
5. Full code
6. Final Questions

### TL;DR

The overall term of difficulty refers to how much work has to be done for a node to find a hash that is smaller than the target. There is one value stored in a block that talks about difficulty — `bits`. In order to calculate the `target` value that the hash, when converted to a hex value has to be less than, we use the `bits` field and run it through an equation that returns the target. We then use the `target` to calculate `difficulty`, where `difficulty` is only a number for a human to understand how difficult the proof of work is for that block.

If you read on, I go through how the blockchain determines what target number the mined block’s hash needs to be less than to be valid, and how that target is calculated.

### Calculate Target from Bits

In order to go through Bitcoin’s PoW, I need to use the values on actual blocks and explain the calculations, so a reader can verify all this code themselves. To start, I’m going to grab a random block number to work with and go through the calculations using that.

```>>>import random
>>> random.randint(0, 493928)
111388```

Block number 11138 it is! Back in time to March of 2011 we go.