1. Block size in Bitcoin vs Ethereum

• Bitcoin: Each block has a fixed size of 1 MB (megabyte). So the number of transactions per block depends on how many fit inside 1 MB of data.
• Ethereum: Instead of measuring block size in bytes, Ethereum measures it in gas.
  • Currently, the maximum capacity of a block is 30 million gas.
  • This is not “30 MB” or “30 GB”; it is purely a unit of computational effort.

2. Why Ethereum uses gas-based block limits

Ethereum transactions are not uniform. A simple ETH transfer uses little gas (21,000), but a smart contract interaction can consume hundreds of thousands of gas.

So the Ethereum community decided:

• Let’s define block capacity in terms of gas used instead of raw data size.
• This ensures that no single extremely heavy transaction (like a contract requiring 40 million gas) can overload the network.

If blocks were only defined in bytes (like Bitcoin), you could fit a massive, computation-heavy contract inside a single block and slow down the whole network.

3. Gas consumption of transactions

• Simple ETH transfer → always 21,000 gas.
• ERC-20 transfer → ~50,000 to 65,000 gas.
• Complex smart contracts → 200,000–1,000,000+ gas depending on logic.

This means the number of transactions per block depends on how much gas each transaction consumes.