Collision Domain Count Calculator

Work out collision domain count instantly with clear inputs, formula shown and shareable results.

About Collision Domain Count Calculator

Each switch port is its own collision domain, so a full-duplex switched port has none at all in practice. A hub, by contrast, joins all of its stations into one shared half-duplex domain where CSMA/CD applies and bandwidth is divided between the stations. Counting domains is the classic way of showing why replacing hubs with switches multiplies usable capacity.

Formula

Domain count

collision domains = switch ports in use; nodes = (ports - hubs) + hubs x stations per hub; shared bandwidth per node = link speed / stations per hub

Worked example

With the default values below, Collision Domain Count Calculator returns Collision domains of 24. Change any input above and every figure here is recalculated in your browser.

Inputs used

Switch ports in useminimum 1: 24
Hubs or repeaters attachedminimum 1: 3
Stations per hubminimum 2: 8
Link speedminimum 1 Mbps: 100 Mbps

Results

Collision domains: 24
Attached nodes: 45
Nodes sharing each hub domain: 8
Effective bandwidth per shared node: 12.500 Mbps

Step by step

1Values used
2Domain count
3Collision domains= 24
4Attached nodes= 45
5Nodes sharing each hub domain= 8
6Effective bandwidth per shared node= 12.500 Mbps

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 3 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 many broadcast domains does a switch create?

One per VLAN, regardless of port count. Collision domains and broadcast domains are separate concepts and only routing splits the latter.

Do collisions still occur on modern networks?

Not on full-duplex links, where CSMA/CD is disabled entirely. Collisions only appear where legacy half-duplex hubs or misnegotiated duplex settings remain.