Block Time
Block time is the average interval between successive blocks being added to a blockchain, determining the baseline latency between transaction submission and initial on-chain confirmation and reflecting the throughput and design priorities of the underlying consensus mechanism.
Block time is one of the most fundamental parameters of a blockchain's user experience. A network with a 12-second block time (Ethereum) will confirm a new transaction at most 12 seconds after it is included by a block proposer, while a network with a 400-millisecond block time (Solana) delivers sub-second inclusion. For applications like decentralized exchanges, faster block times reduce the window of price uncertainty between order placement and execution.
In proof-of-work blockchains, block time is regulated probabilistically by adjusting mining difficulty. Bitcoin targets a ten-minute average block time through a two-week difficulty adjustment period. If blocks are found faster than target, difficulty increases; if slower, it decreases. This mechanism keeps block time stable on average despite variations in total network hash rate.
In proof-of-stake blockchains, block time is typically deterministic. Ethereum post-Merge produces a block every 12 seconds by design: the slot time is fixed at 12 seconds, with each slot assigned to a validator who must propose a block. If the assigned validator is offline, the slot is skipped and the next 12-second slot proceeds. This produces a slightly irregular actual block time but a tightly bounded maximum inter-block interval.
Solana targets approximately 400 milliseconds per slot, enabled by its proof of history mechanism that pre-orders transactions before consensus attestation begins. Near Protocol, Avalanche's C-chain, and other high-performance chains target one- to two-second block times.
Block time interacts with block size to determine throughput. A blockchain can increase transactions per second by decreasing block time, increasing block size (or gas limit), or both. Decreasing block time too aggressively strains network propagation: if blocks are produced faster than they can propagate across the network, validators will frequently build on stale tips, increasing the rate of orphaned blocks and reducing security. The optimal block time for a given network depends on the median propagation delay across its validator set.