MAC Table Size Calculator
Size a switch CAM table from host count and VLAN membership, and see how long before growth fills it.
About MAC Table Size Calculator
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).
Worked example
With the default values below, MAC Table Size Calculator returns MAC Entries Required of 3,856. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Unique MAC addressesminimum 1 MACs
- 3,000 MACs
- VLANs each MAC appears inaccepts 1–16 VLANs
- 1.2 VLANs
- Static and reserved entriesminimum 0 entries
- 256 entries
- Hardware MAC table sizeminimum 1 entries
- 32,768 entries
- Annual host growthaccepts 0–200 %
- 15 %
Results
- 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
- 1Values used
- 2MAC Table Size
- 3Time to exhaustion
- 4MAC Entries Required= 3,856
- 5Dynamically Learned Entries= 3,600
- 6MAC Table Used= 11.77
- 7Free Entries= 28,912
- 8Years Until the Table Fills= 15.3
- 9Unknown-Unicast Flooding Risk= Low — comfortable headroom
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges 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
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.