Collision Domain Calculator

Count collision domains in a mixed switch and hub topology, with CSMA/CD slot time and cable diameter limits.

About Collision Domain Calculator

CSMA/CD requires a sender to still be transmitting when a collision returns, which is why the minimum frame is 64 bytes and the slot time 512 bit times. That fixes a round-trip propagation budget and therefore a maximum network diameter, before repeater and transceiver latency is subtracted. Half-duplex Ethernet is rare but not gone — a duplex mismatch turns a switch port back into a collision domain, and the resulting late collisions and FCS errors look like a failing cable rather than a configuration fault.

Formulas

Collision Domain

Each full-duplex switch port is its own collision domain; each hub or half-duplex segment is one shared domain whose bandwidth divides among its stations.

slot time: 512 bit times — the 64-byte minimum frame, which is what makes collisions detectable
propagation budget: Half the slot time × 1.95 × 10^8 m/s signal velocity in copper
512 bit times: 51.2 µs at 10 Mbps, 5.12 µs at 100 Mbps

CSMA/CD timing

slot time = 512 ÷ line rate; cable budget = (slot time ÷ 2) × 0.65c.

Worked example

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

Inputs used

Switch portsaccepts 1–512 ports: 24 ports
Hubs or half-duplex segmentsaccepts 0–64 segments: 2 segments
Stations per shared segmentaccepts 1–128 stations: 8 stations
Shared segment speedone of 10 Mbps, 100 Mbps, 1000 Mbps: 10 Mbps

Results

Collision Domains: 24
Stations Sharing Bandwidth: 16
CSMA/CD Slot Time: 51.200 µs
Propagation Budget: 4,992 m
Bandwidth per Shared Station: 1.250 Mbps
Assessment: Contended — 16 station(s) share 2 half-duplex segment(s) at 1.3 Mbps each

Step by step

1Values used
2Collision Domain
3CSMA/CD timing
4Collision Domains= 24
5Stations Sharing Bandwidth= 16
6CSMA/CD Slot Time= 51.200 µs
7Propagation Budget= 4,992 m
8Bandwidth per Shared Station= 1.250 Mbps
9Assessment= Contended — 16 station(s) share 2 half-duplex segment(s) at 1.3 Mbps each

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 is Collision Domain calculated?

Each full-duplex switch port is its own collision domain; each hub or half-duplex segment is one shared domain whose bandwidth divides among its stations. CSMA/CD requires a sender to still be transmitting when a collision returns, which is why the minimum frame is 64 bytes and the slot time 512 bit times. That fixes a round-trip propagation budget and therefore a maximum network diameter, before repeater and transceiver latency is subtracted.

Why does Collision Domain matter?

Half-duplex Ethernet is rare but not gone — a duplex mismatch turns a switch port back into a collision domain, and the resulting late collisions and FCS errors look like a failing cable rather than a configuration fault.

What values do I need to enter?

This calculator takes 4 inputs: Switch ports, Hubs or half-duplex segments, Stations per shared segment, Shared segment speed. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Why do I still see collision counters on a modern switch?

A non-zero late-collision counter on a gigabit port almost always means a duplex mismatch: one side auto-negotiated full duplex, the other is hard-coded to half. Fix the pair, do not replace the cable.