What is block size?

The size of a block equals the amount of data it stores. And just like any other container, a block can only hold so much information. The largest amount of data a blockchain block can hold is referred to as the block size limit.

Blockchain size limits are small by modern data storage standards, but crypto transactions are very lightweight, when it comes to data storage. Bitcoin’s block size is limited to 1 MB, but this small amount of data is enough to store over 2000 transactions.

Block size essentials

• The amount of data stored in a block.
• The largest amount of data a block can hold is called the block size limit.
• A larger block size limit enables a higher transaction-per-second rate, but may result in stales and temporary chain splits.
• Different blockchains have different block size limits.
• Ethereum does not have a block size limit but a gas limit.

A brief history of block size

When Satoshi Nakamoto mined Bitcoin’s genesis block in 2009, there was no explicit block size limit. He introduced the block size limit a year later, when he realized it was the only way to prevent crypto miners from creating blocks larger than other miners could accept. He incorporated a piece of code that limited the block size to 1 MB.

But some people believe that Nakamoto’s limit was unwise. They argue that Bitcoin’s block size limit should be increased to enable a higher transactions-per-second rate. Bitcoin has often faced criticism for its transaction rate, which is currently around 4 transactions per second (on the base chain). At this speed, not all transactions make it into a block during busy periods and have to wait in queue for the next block.

Actual block sizes at the time were much smaller than 1 MB, and Nakamoto likely did not anticipate that blocks would ever become overcrowded with data. Also, if the limit were higher, nodes would need a faster internet connection. If one node wanted to upload a large block to its peers, the potential delays in block transfers could cause the system to go out of sync. This would result in temporary chain splits (called forks), before everyone got back on track.

The debate about increasing Bitcoin’s block size limit started troubling the cryptocurrency world in 2015. It finally came to an end in mid-2017 when a group of miners, investors and activists forked the Bitcoin blockchain. The block size limit of the newly-created fork was increased from 1 to 8 MB, and this is essentially how Bitcoin Cash came to be. Bitcoin Cash then continued on a path where they would regularly increase block size to cater to their user’s needs.

However, technically, Bitcoin’s block size may actually be greater than 1 MB, and many blocks published nowadays are larger than that. This is thanks to an upgrade to the Bitcoin protocol called Segregated Witness, or SegWit for short. SegWit replaced the concept of block size with block weight and virtually increased the size by four times (in theory, at least, since real blocks rarely approach that size).

Major protocols and their block size limits

Bitcoin

1 MB

Bitcoin Cash

32 MB*

Litecoin

1 MB

Ethereum

/

Block size (BCH continues to increase block size)

Bitcoin’s block size limit of 1 MB is shared by the Litecoin protocol. But because Litecoin’s stingy block time is four times shorter, its transaction-per-second rate is four times higher than that of Bitcoin. You can see in the table above that Bitcoin Cash has by far the largest block size of the networks we have listed. Ethereum, by contrast, is in a category all by itself. There, the largest size of a block is not determined by a block size limit, but rather by the so-called gas limit.

When there is a lot of traffic on a blockchain network, blocks sometimes get filled to their limit. This means that some transactions can’t be included in one block and have to wait for the next one. This problem is being addressed by various blockchain scaling solutions.