DC Gateway Capacity Calculator
Count the border gateways a data centre needs based on throughput, NAT session scale and redundancy.
About DC Gateway Capacity Calculator
Gateways are limited by two independent budgets — packets per second expressed as throughput, and stateful session table size — and the binding constraint is whichever needs more boxes. Redundancy units are then added on top so a failure or upgrade does not reduce capacity below demand. Session tables usually fill before bandwidth does on NAT and firewall gateways, and this shows which of the two is driving the hardware count.
Formula
DC Gateway Capacity
active gateways = max(throughput ÷ per-gateway throughput, sessions ÷ per-gateway sessions), rounded up; deployed = active + redundant units.
- N+1
- One spare unit beyond the active set
- sessions
- Concurrent stateful flows the gateway must track
Worked example
With the default values below, DC Gateway Capacity Calculator returns Gateways to Deploy of 4. Change any input above and every figure here is recalculated in your browser.
Inputs used
- North-south throughputaccepts 0.1–400,000 Gbps
- 200 Gbps
- Concurrent NAT or flow sessionsaccepts 1,000–2,000,000,000 sessions
- 2,000,000 sessions
- Throughput per gatewayaccepts 0.1–40,000 Gbps
- 80 Gbps
- Session capacity per gatewayaccepts 1,000–2,000,000,000 sessions
- 4,000,000 sessions
- Redundancy modelone of N+1, N+2, No redundancy
- N+1
Results
- Gateways to Deploy
- 4 units
- Active Gateways Required
- 3 units
- Utilisation of the Active Set
- 83.33%
- Spare Throughput
- 120.0 Gbps
- Gateways Needed for Sessions Alone
- 1 units
Step by step
- 1Values used
- 2DC Gateway Capacity
- 3Gateways to Deploy= 4 units
- 4Active Gateways Required= 3 units
- 5Utilisation of the Active Set= 83.33
- 6Spare Throughput= 120.0 Gbps
- 7Gateways Needed for Sessions Alone= 1 units
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 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 is DC Gateway Capacity calculated?
active gateways = max(throughput ÷ per-gateway throughput, sessions ÷ per-gateway sessions), rounded up; deployed = active + redundant units. Gateways are limited by two independent budgets — packets per second expressed as throughput, and stateful session table size — and the binding constraint is whichever needs more boxes. Redundancy units are then added on top so a failure or upgrade does not reduce capacity below demand.
Why does DC Gateway Capacity matter?
Session tables usually fill before bandwidth does on NAT and firewall gateways, and this shows which of the two is driving the hardware count.
What values do I need to enter?
This calculator takes 5 inputs: North-south throughput, Concurrent NAT or flow sessions, Throughput per gateway, Session capacity per gateway, Redundancy model. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.