How much data will a 12550824 x 12550824 world take up in Minecraft?

Are you trying to build the Earth (it's actually about 22584607 by 22584607 metres surface area, about 3.2 times what you're asking about)? Well, the block height can take 256 values and each block uses at most ~2.5 bytes, so it's 12550824*12550824*256*2.5 = 100,814,837,170,544,640 bytes, aka 100,815,000 GB, aka 101 petabytes. You're going to need a bigger disk ;)

Compression could reduce this somewhat, but depends heavily on the complexity of the world. So unless your world is ultraflat it's probably still going to be a petabyte or more.

As the existing answers only guess about compression, I estimated the space requirements from an existing server. That server has a 60kx60k world (-30000 to +30000 each X and Z), which is approximately 50% generated, and which needs 28 GB.

Let's assume it needs 56 GB fully generated.

Your world has ~ 210 times the size in X AND Z, so 44,100 times the size. 44,100 * 56 GB makes 24,696,000, so a bit less than 25,000 TB, or 25 PB.

This is way more than achievable with a standard PC, but if you build a supercomputer, it's very well reachable. The IBM Summit has ten times as much, and it's a real bargain at $200M.

Now, let's check how much time you need to generate the world. On my Ryzen 7 3800X, using WorldBorder, I can generate approx. 200 chunks per second. Your world has (12550284²)/(16²) ~ 6.1*10¹¹ chunks. This makes 30 trillion seconds, or 35,606 days. Wow, just 100 years to populate your world after you've assembled the computer to store the data, and started the generation process.

Of course, you'll probably have more than one CPU in your supercomputer. Let's assume you have 64 cores, set up 64 instances of Minecraft, and let each of them generate a part of the world. Assuming the CPUs are faster than mine, this may give you a factor of 100. So your world may actually be done in just 1 year!

In regards to the main question of "How many bytes would a world of this size require?"

The answer to that varies between Minecraft version, the kind of compression involved and the specifics of the world being created, and thus is impossible to get anything more than a ballpark figure for.

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In regards to the secondary question of "Can I fit a world of this size in the 3GB left over on my USB stick", I'd like to demonstrate a different way of proving that 3GB is nowhere near enough using a worked example and some fictitious assumptions to make the maths a bit more appealing.

Let's assume that:

• 1 chunk = 16x16x256 blocks
• 1 chunk requires 1 byte
  • It's probably a lot more, this is a simplification

In which case the question becomes how many bytes would you need to store a 12550824 * 12550824 world...

• 12550824 / 16 is 784426.5
  • The number of chunks along each dimension
• 784426.5 * 784426.5 is 615324933902.25 bytes
  • The number of total chunks, and thus the number of bytes needed to store that many chunks, assuming 1 byte per chunk
• 615324933902.25 / 1024 is 600903255.763916015625 KB
• 600903255.763916015625 / 1024 is 586819.5857069492340087890625 GB
• 586819.5857069492340087890625 / 1024 is 573.06600166694261133670806884766 TB
  • The total number of terabytes, stopping there because that's smaller than 1 petabyte

~574 Terabytes worth of fictional 1-byte chunks.

So assuming that 1 chunk could magically be stored in a single byte (a feat that is, as far as I'm aware, impossible), you're already looking at terabytes of information. In reality, chunks would undoubtedly require more than just a single byte of information, so you'd easily be looking at more than just a few terabytes..

Hence the answer is "Not even close". Either shrink your world or get a bigger USB stick.