Block Gas Limit

The block gas limit is the primary constraint on a blockchain's transaction throughput. It caps the total amount of gas—a unit of computational effort—that can be consumed by all transactions and smart contract executions within a block. A higher limit allows more transactions per block, increasing transactions per second (TPS). However, it is balanced against the need to keep block propagation times low to maintain network consensus.

This limit protects the network by preventing excessively large blocks that could slow down propagation and lead to chain reorganizations. If blocks are too large:

• Node synchronization becomes slower, potentially centralizing the network among nodes with high-performance hardware.
• The risk of uncle blocks or orphaned blocks increases, harming consensus stability. Thus, the limit is a key parameter in the scalability trilemma, balancing scalability with decentralization and security.

Block gas limits are not always static. Networks use different consensus mechanisms for adjustment:

• Ethereum (Pre-1559): Miners could vote to adjust the limit incrementally block-by-block.
• Ethereum (Post-1559): The limit is algorithmically adjusted with a base fee targeting 50% block capacity, making it more predictable.
• Other L1s: May have fixed limits or different governance models for changes. This dynamic nature allows the network to adapt to changing demand.

The limit creates a competitive market for block space. When demand for transactions exceeds the gas available in a block:

• Users must outbid others by offering higher gas prices (priority fees) to get their transactions included.
• This leads to gas auctions and volatile fee markets. The base fee mechanism in EIP-1559 directly ties fee calculation to how full the previous block was relative to the gas limit, creating a more efficient pricing model.

While often conflated, block gas limit and block size are distinct but related concepts. Gas measures computational complexity, while size measures data in bytes. A block's physical size is a function of the transactions it contains and their gas usage. Networks like Ethereum use gas to price different types of operations (e.g., storage writes cost more than simple adds), making gas a more nuanced measure of resource consumption than raw data size alone.

As a concrete example, the Ethereum mainnet has a target gas limit of 15 million gas and a maximum gas limit of 30 million gas per block (as of late 2023). A standard ETH transfer consumes 21,000 gas, so a block at the target limit could theoretically hold ~714 simple transfers. A complex Uniswap swap might consume 150,000+ gas, significantly reducing the number of transactions per block. This illustrates the trade-off between transaction complexity and network throughput.

Ethereum's block gas limit is a dynamic parameter set by miners/validators through a voting mechanism. It is not a fixed constant and can change over time to balance network throughput and node resource requirements. The limit is enforced at the consensus layer and directly impacts the maximum number of transactions per block. Historically, it has increased from 8 million gas to over 30 million gas, with a current target of 15 million gas per block post-EIP-1559.

As an Ethereum Layer 2 scaling solution, Polygon PoS has a significantly higher block gas limit than Ethereum Mainnet to enable greater throughput. This is a core part of its scaling design. Key characteristics include:

• Higher Limit: Allows for more transactions per block, reducing fees.
• Fixed Parameter: The limit is a protocol constant, not dynamically voted on by validators.
• Heimdall Layer: The checkpointing layer to Ethereum also has its own gas limit for proof submission transactions.

Optimistic rollups like Arbitrum One and Optimism handle gas limits differently. They have two primary limits:

• L2 Block Gas Limit: The maximum computational work for a block within the rollup's own sequencer. This is typically very high to maximize throughput.
• L1 Data/Calldata Limit: The critical constraint is the amount of transaction data that can be posted in a single batch to the Ethereum Mainnet (L1). This L1 gas cost for data availability is the ultimate bottleneck for rollup capacity and cost.

The Avalanche C-Chain, which is EVM-compatible, uses a block gas limit as a protocol parameter. Unlike Ethereum's miner-voted system, it is set by the network's validators through the AvalancheGo client configuration and governance. The limit is designed to be high to support the network's sub-second finality and high throughput goals. Transactions that exceed the per-block limit will fail, similar to other EVM chains.

BNB Smart Chain implemented a high block gas limit (e.g., 140 million gas) as a key feature to offer lower transaction fees compared to Ethereum at the time of its launch. This is a fixed parameter in the Geth client fork used by BSC. The high limit allows more transactions per block but requires validators to run more powerful hardware. It represents a deliberate trade-off between decentralization (higher node requirements) and scalability (lower user fees).

ZK-Rollups like zkSync Era and Starknet are constrained by the proving computational cost on L1, not a traditional gas limit for L2 execution. Their throughput is limited by:

• Prover Capacity: The time and cost to generate a zero-knowledge proof (ZKP) for a block of transactions.
• L1 Verification Gas: The gas required to verify the proof on Ethereum.
• L1 Data Availability: For validiums/volitions, the cost of posting data to a Data Availability Committee (DAC) or Ethereum calldata.

The unit of computational work on the Ethereum Virtual Machine (EVM). Every operation (e.g., adding numbers, storing data) has a fixed gas cost. The Block Gas Limit defines the maximum total gas that can be consumed by all transactions in a block.

• Purpose: Measures and prices computational effort.
• Paid in: The network's native currency (e.g., ETH).
• Key Relation: Sum of all transaction gas in a block ≤ Block Gas Limit.

The minimum gas price per unit required for a transaction to be included in a block, burned by the protocol. It is algorithmically adjusted block-by-block based on network congestion relative to the target gas limit.

• Mechanism: If the previous block was >50% full, the base fee increases; if <50%, it decreases.
• Purpose: Dynamically regulates demand to keep block size near the target, making the Block Gas Limit a flexible ceiling.

The maximum amount of gas a user is willing to spend on a single transaction. It is set by the transaction sender and acts as a safety cap to prevent runaway costs from bugs or infinite loops.

• User Control: Protects against unexpectedly high costs.
• Block Constraint: A transaction's gas limit must be ≤ the Block Gas Limit.
• Unused Gas: Is refunded to the sender, ensuring they only pay for work done.

Transactions Per Second, a measure of network capacity. The Block Gas Limit is a primary determinant of theoretical maximum TPS.

• Calculation: Max TPS ≈ Block Gas Limit / (Average Gas per Tx * Block Time).
• Scalability Trade-off: Increasing the limit raises throughput but also increases block size, potentially harming decentralization by raising hardware requirements for validators.

The network participant responsible for proposing new blocks. They have direct influence over the Block Gas Limit in some consensus models.

• Historical Role (PoW): Miners could vote to adjust the limit incrementally.
• Current Role (PoS): Validators follow protocol rules; the limit is adjusted algorithmically via EIP-1559, with the base fee mechanism managing congestion.

The Ethereum Improvement Proposal that overhauled the fee market. It introduced the base fee and made the Block Gas Limit a flexible parameter with a target and a maximum (2x the target).

• Target Gas Limit: The ideal block size the protocol aims for (e.g., 15 million gas).
• Max Gas Limit: The absolute cap (e.g., 30 million gas). This is the hard Block Gas Limit that cannot be exceeded.