A slot, on the Cardano blockchain, is a fixed unit of time, one second long since the Shelley phase began in 2020, during which a block may be added to the chain. Slots are grouped into epochs of 432,000 slots, or five days. Under the Ouroboros Praos protocol, each stake pool privately runs a lottery for every slot, weighted by its stake; a pool that wins is a slot leader and may produce a block. The protocol is tuned so that only a small share of slots, about one in twenty, has a leader, which means a block appears roughly every 20 seconds on average and empty slots are normal. Sometimes two pools win the same slot, and the network chooses between their blocks. Time slots are also used in other proof-of-stake chains: an Ethereum slot, for example, lasts 12 seconds and has exactly one assigned proposer.
Last updated 13 Nov 2026