Work out arp table size instantly with clear inputs, formula shown and shareable results.
A router or SVI keeps one ARP entry per reachable host in each attached subnet, so the table scales with the size of the broadcast domain. Every entry ages out after the aging timer and must be re-resolved, which produces a steady background ARP rate of entries divided by the timer. Large flat VLANs push both table memory and that refresh rate up, and both have hard platform limits.
ARP table load
entries = active hosts; memory = entries x bytes per entry; refresh rate = entries / aging time
New hosts cannot be resolved, so traffic to them is dropped or punted to the CPU. On hardware platforms this also exhausts the adjacency table and causes forwarding to fall back to software.
A longer timer cuts refresh traffic but keeps stale entries after a host moves. Four minutes is a common compromise; lengthen it only on stable, statically addressed segments.