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I am not asking about block weight. I hear many people saying that the actual Bitcon block size is 2mb. However, in any block explorer, I can see that blocks are around 1200kb. This is very confusing, where is the cap?

  • In actual block explorers you can also occasionally see blocks larger than 2MB. – Jannes Aug 17 at 12:03
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The cap on the block size excluding witness data is 1000000 bytes.

There is no cap on block size including witness data. Instead, there is a limit on the block weight at 4000000 weight units. Depending on the composition of the block, 4000000 WU can correspond to anything from 1000000 bytes to close to 4000000 bytes (including witness data).

But a block with 4000000 WU is "full" - regardless of whether it's 1 MB or 3.8 MB of data.

  • Why at 4000000 specifically? If that was chosen by consensus fork, why that number? – Josh Aug 21 at 7:37
  • The factor 4 for weight/ood byte was chosen as a tradeoff between encouraging UTXO consumption and enabling more network bandwidth. The limit 4000000 must be exactly the chosen factor times the old 1000000 limit, or it wouldn't be a soft fork (we must guarantee that every valid block without witnesses still stays below the limit). – Pieter Wuille Aug 21 at 15:25

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