PAT Port Capacity Calculator

Work out pat port capacity instantly with clear inputs, formula shown and shareable results.

About PAT Port Capacity Calculator

Carrier-grade NAT assigns each subscriber a fixed block of ports so logging stays deterministic. Capacity is then simply the total usable ports across the public pool divided by the block size: a 512-port block on 64,512 usable ports gives 126 subscribers per address. Larger blocks improve application compatibility but cut subscriber density proportionally.

Formula

PAT capacity

total ports = public IPs x usable ports; subscribers = floor(total ports / ports per subscriber)

Worked example

With the default values below, PAT Port Capacity Calculator returns Subscribers supported of 504. Change any input above and every figure here is recalculated in your browser.

Inputs used

Public IPs availableminimum 1: 4
Ports allocated per subscriberminimum 64: 512
Subscribers to supportminimum 1: 2,000
Usable ports per IPaccepts 1,024–65,535: 64,512

Results

Subscribers supported: 504
Total translation ports in the pool: 258,048
Pool utilisation at the target subscriber count: 396.83%
Extra public IPs needed: 12

Step by step

1Values used
2PAT capacity
3Subscribers supported= 504
4Total translation ports in the pool= 258,048
5Pool utilisation at the target subscriber count= 396.83
6Extra public IPs needed= 12

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 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 many ports does a subscriber really need?

Light browsing fits in a few hundred, but video, streaming and many devices per household push it higher. Providers commonly deploy 512 to 2048 ports per subscriber.

What breaks when the block is too small?

Web pages stall on some assets, and applications that open many parallel sockets fail intermittently, which is hard to diagnose from the client side.