Work out randomisation block size instantly with clear inputs, formula shown and shareable results.
Block randomisation guarantees balance at the end of each block. Two arms with a multiplier of two gives blocks of four, so 120 participants need 30 blocks and imbalance can never exceed two. Small blocks balance tightly but become predictable to unblinded staff, which is why variable block sizes are usually preferred.
Block size
size = arms x multiplier
Blocks needed
blocks = ceil(total participants / block size)
Because the last allocation in a block of two is fully predictable, which lets unblinded staff guess assignments and introduces selection bias.
Randomly alternating between, say, blocks of four and six. This preserves balance while defeating prediction.