Block Height Explained
Last Updated: 30th October 2018
Block height, as it pertains to cryptocurrency, refers to the current number block in a blockchain. The genesis block, which is the very first block in any blockchain, has a block height equal to zero. Therefore, the block height is always a positive integer greater than zero.
A block is simply a collection of individual transactions such as: Alice sends 1 bitcoin to Bob. When transactions are bundled together, the bundle then becomes referred to as a block. As well as containing a batch of transactions, each block will contain a cryptographic hash and a time stamp in order to ensure the integrity of the block, as well as the entire blockchain.
As of writing this article, the current block height of the Bitcoin blockchain is 537,055. The Bitcoin protocol has a block generation time of roughly 10 minutes. What this means is, new blocks are added to the Bitcoin blockchain at an approximate rate of 1 block every 10 minutes.
Conversely, the current block height of the Ethereum protocol is 6,158,794. Ethereum’s block generation time of 15 seconds, i.e. new blocks being added to the Ethereum blockchain at a rate of 1 block every 15 seconds, means that even though the Ethereum protocol was launched in 2015, whereas the Bitcoin protocol was launched in 2009, its quicker block generation time has resulted in it having a much larger block height number.
However, a larger block height number does not necessarily result in a blockchain being more secure. The proof-of-work, i.e. that amount of resources expended by individuals in producing blocks, is used as an indicator in determining how secure a blockchain is. Currently, despite Bitcoin having a smaller block height number than Ethereum, it is significantly more secure than Ethereum, as is indicated by the current hash rate of both protocols. The more proof-of-work expended in producing blocks for a blockchain, the more secure it is.
Furthermore, a larger block height number indicates a more recent addition of a block to the blockchain. For example, a block height number of 200 compared to 100, indicates that the transactions in block height number 200 are more recent than that of the transactions block height number 100.
Confirmations refer to the number of blocks that have been added to the blockchain after a block containing a user’s transaction has been successfully added. To illustrate, consider that Alice’s transaction to Bob was added in block height number 10. When another block is added on top of Alice’s block, at block height number 11, this is said to count as 1 confirmation. When another block is subsequently added, with a block height number of 12, this counts as 2 confirmations.
Merchants and exchanges will typically wait for a certain number of confirmations before accepting cryptocurrency payments in order to prevent what is known as a double-spend attack; an attack in which a bad actor attempts to spend funds more than once. The more confirmations that a transaction has, the more expensive it becomes for a bad actor to attempt a double-spend attack.
In the Bitcoin protocol, 6 confirmations is considered the point at which a double-spend attack becomes unlikely to occur. However, the number 6 is an arbitrary figure, and as such, cryptocurrency exchanges such as Coinbase, Binance and Kucoin, typically require a much larger number of confirmations before clearing the movement of funds through their platform.
To conclude, block height refers to the current block number in a blockchain, with the genesis block having a block height number of 0.
Blockchains with a quicker block generation time tend to have a larger current block height number, however, this does not necessarily result in the blockchain being more secure.
Subsequent blocks being added to the block containing a user’s transaction is referred to as confirmations. The larger the number of confirmations a transaction has, the more expensive it becomes for an attacker to perform a double-spend attack.