MAC Table Size Calculator
Size a switch CAM table from host count and VLAN membership, and see how long before growth fills it.
Inputs
MAC Entries Required
3,856
Dynamically Learned Entries
3,600
MAC Table Used
11.77%
Free Entries
28,912
Years Until the Table Fills
15.3
Unknown-Unicast Flooding Risk
Low — comfortable headroom
Step by step
Values used
Unique MAC addresses = 3,000 MACs; VLANs each MAC appears in = 1.20 VLANs; Static and reserved entries = 256 entries; Hardware MAC table size = 32,768 entries; Annual host growth = 15 %
MAC Table Size
Entries = hosts × VLANs per host + static entries, because a MAC is learned once per VLAN it appears in.
Time to exhaustion
years to full = ln(capacity ÷ entries) ÷ ln(1 + growth).
MAC Entries Required
= 3,856
Dynamically Learned Entries
= 3,600
MAC Table Used
= 11.77
Free Entries
= 28,912
Years Until the Table Fills
= 15.3
Unknown-Unicast Flooding Risk
= Low — comfortable headroom
How it works
A switch learns a MAC per VLAN, not per box, so a trunked hypervisor with interfaces in several VLANs consumes several entries. Compound growth against the hardware limit gives the year the table runs out, which is the useful figure for a refresh plan. A full CAM table does not error — it floods unknown unicast out every port in the VLAN, so throughput quietly collapses and traffic that should be switched becomes visible to every host in the segment.
Formulas
MAC Table Size
Entries = hosts × VLANs per host + static entries, because a MAC is learned once per VLAN it appears in.
- VLANs per host
- Average VLAN memberships per MAC — above 1 for trunked servers and hypervisors
- static entries
- Manually configured and platform-reserved entries
- years to full
- ln(capacity ÷ current) ÷ ln(1 + growth rate)
Time to exhaustion
years to full = ln(capacity ÷ entries) ÷ ln(1 + growth).
Frequently Asked Questions
How is MAC Table Size calculated?
Entries = hosts × VLANs per host + static entries, because a MAC is learned once per VLAN it appears in. A switch learns a MAC per VLAN, not per box, so a trunked hypervisor with interfaces in several VLANs consumes several entries. Compound growth against the hardware limit gives the year the table runs out, which is the useful figure for a refresh plan.
Why does MAC Table Size matter?
A full CAM table does not error — it floods unknown unicast out every port in the VLAN, so throughput quietly collapses and traffic that should be switched becomes visible to every host in the segment.
What values do I need to enter?
This calculator takes 5 inputs: Unique MAC addresses, VLANs each MAC appears in, Static and reserved entries, Hardware MAC table size, Annual host growth. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
What does MAC aging do to these numbers?
Aging (300 s by default) releases entries for hosts that go quiet, so the steady-state count is usually below the raw host count. Size for the peak, though: a boot storm or a scan can populate every entry at once.