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Oversubscription Ratio Calculator

Compare access bandwidth with uplink bandwidth to get the oversubscription ratio and the uplinks a target needs.

About Oversubscription Ratio Calculator

The ratio assumes every access port could transmit at line rate simultaneously, which is the worst case the fabric must survive. A 1:1 result is non-blocking; 3:1 is the common enterprise compromise; storage and AI fabrics are usually built at 1:1. Oversubscription is the single number that predicts incast drops, and it is far cheaper to add uplinks at build time than to re-cable a live rack.

Formula

Oversubscription Ratio

oversubscription = (access ports × access speed) ÷ (uplink ports × uplink speed); uplinks for a target = access capacity ÷ (target × uplink speed), rounded up.

access capacity
Aggregate speed of all server-facing ports
uplink capacity
Aggregate speed of all fabric-facing ports

Worked example

With the default values below, Oversubscription Ratio Calculator returns Oversubscription Ratio of 2:1. Change any input above and every figure here is recalculated in your browser.

Inputs used

Access portsaccepts 1–1,024 ports
48 ports
Access port speedaccepts 0.1–800 Gbps
25 Gbps
Uplink portsaccepts 1–256 ports
6 ports
Uplink port speedaccepts 0.1–800 Gbps
100 Gbps
Target ratio (x:1)accepts 1–30 x
3 x

Results

Oversubscription Ratio
2:1
Ratio as a Number
2.000
Access Capacity
1,200.0 Gbps
Uplink Capacity
600.0 Gbps
Uplinks Needed for the Target
4 ports
Assessment
Comfortable for general server workloads

Step by step

  1. 1Values used
  2. 2Oversubscription Ratio
  3. 3Oversubscription Ratio= 2:1
  4. 4Ratio as a Number= 2.000
  5. 5Access Capacity= 1,200.0 Gbps
  6. 6Uplink Capacity= 600.0 Gbps
  7. 7Uplinks Needed for the Target= 4 ports
  8. 8Assessment= Comfortable for general server workloads

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, 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 Oversubscription Ratio calculated?

oversubscription = (access ports × access speed) ÷ (uplink ports × uplink speed); uplinks for a target = access capacity ÷ (target × uplink speed), rounded up. The ratio assumes every access port could transmit at line rate simultaneously, which is the worst case the fabric must survive. A 1:1 result is non-blocking; 3:1 is the common enterprise compromise; storage and AI fabrics are usually built at 1:1.

Why does Oversubscription Ratio matter?

Oversubscription is the single number that predicts incast drops, and it is far cheaper to add uplinks at build time than to re-cable a live rack.

What values do I need to enter?

This calculator takes 5 inputs: Access ports, Access port speed, Uplink ports, Uplink port speed, Target ratio (x:1). The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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