NAT Gateway Capacity
Estimate NAT gateway connection and bandwidth capacity for outbound cloud traffic.
About NAT Gateway Capacity
A NAT gateway's maximum concurrent connections to any single destination IP/protocol/port combination is limited by the number of ephemeral source ports available, multiplied by how many public IP addresses the gateway can use: connections = ports_per_ip × elastic_ips. Each additional public IP effectively multiplies the available port pool, since ports are tracked per source-IP, allowing the gateway to open many more simultaneous connections to the same busy destination before running into port exhaustion. Bandwidth is a separate, independent limit — the gateway's throughput ceiling regardless of connection count.
Formula
Max concurrent connections
connections = ports_per_ip × elastic_ips
- p
- Usable ephemeral ports per IP (~64,000 typical)
- n
- Number of elastic/public IPs attached
Worked example
With the default values below, NAT Gateway Capacity returns Max Concurrent Connections (per destination) of 64,000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of Elastic/Public IPsaccepts 1–8
- 1
- Available Ports per IPaccepts 1–65,536
- 64,000
- Gateway Bandwidth Limitminimum 0.1 Gbps
- 45 Gbps
Results
- Max Concurrent Connections (per destination)
- 64,000
- Gateway Bandwidth Limit
- 45.0 Gbps
- Gateway Bandwidth Limit
- 45,000 Mbps
Step by step
- 1Values used
- 2Max concurrent connections
- 3Max Concurrent Connections (per destination)= 64,000
- 4Gateway Bandwidth Limit= 45.0 Gbps
- 5Gateway Bandwidth Limit= 45,000 Mbps
Assumptions and limits
- The result is derived only from the 3 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 1 decimal place, 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
Why does adding more Elastic IPs increase connection capacity?
NAT translates each outbound connection to a unique combination of (public IP, source port). With only one public IP, you're limited to that IP's ~64,000 usable ports per destination; adding a second IP effectively doubles the available port pool for connections to the same busy destination.
What causes 'port exhaustion' errors on a NAT gateway?
When too many simultaneous connections are made to the same destination IP and port (e.g. a single popular API endpoint) and the available port pool for that destination is used up, new connection attempts fail until existing ones close and free up a port — adding more elastic IPs directly mitigates this.
Does this port limit apply separately per destination?
Yes — the port limit is per unique (destination IP, destination port, protocol) tuple, not a global cap. A NAT gateway can typically support many multiples of this connection count in aggregate as long as traffic is spread across many different destinations rather than concentrated on one.
Is bandwidth or connection count usually the binding constraint?
It depends on the workload — many short-lived connections to few destinations (e.g. web scraping a single site) tend to hit port limits first, while sustained large data transfers tend to hit the bandwidth ceiling first. Both should be checked independently, as this calculator does.