ARP Table Size Calculator

Work out arp table size instantly with clear inputs, formula shown and shareable results.

About ARP Table Size Calculator

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.

Formula

ARP table load

entries = active hosts; memory = entries x bytes per entry; refresh rate = entries / aging time

Worked example

With the default values below, ARP Table Size Calculator returns ARP entries held of 1,200. Change any input above and every figure here is recalculated in your browser.

Inputs used

Active hosts in the broadcast domainminimum 1: 1,200
VLANs terminated on the deviceminimum 1: 12
Memory per ARP entryminimum 16 bytes: 96 bytes
ARP aging timerminimum 0.5 min: 4 min

Results

ARP entries held: 1,200
Control-plane memory for the table: 112.50 KiB
Refresh ARP packets per second: 5.00
Average entries per VLAN: 100.0

Step by step

1Values used
2ARP table load
3ARP entries held= 1,200
4Control-plane memory for the table= 115,200
5Refresh ARP packets per second= 5.00
6Average entries per VLAN= 100.0

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What happens when the ARP table fills?

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.

Should I lengthen the aging timer?

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.