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Session Capacity Calculator

Use Little's law to turn a new-session rate and session lifetime into a firewall session table size.

About Session Capacity Calculator

Little's law says the population of a stable system equals arrival rate times residence time, so concurrent sessions follow directly from the new-session rate and how long sessions live. Inverting it gives the arrival rate at which the table fills. Firewalls and NAT gateways fail by exhausting the session table long before they run out of throughput, and idle-timeout settings drive residence time far more than user behaviour does.

Formulas

Session Capacity

concurrent sessions = new sessions per second × average session duration (Little's law).

new sessions per second
Arrival rate λ
average session duration
Mean time in system W
concurrent sessions
Population L = λW

Rate the table can sustain

max sustainable rate = table size ÷ average duration

table size
Maximum entries the device holds

Worked example

With the default values below, Session Capacity Calculator returns Concurrent Sessions of 120,000. Change any input above and every figure here is recalculated in your browser.

Inputs used

New sessions per secondminimum 0 sessions/s
2,000 sessions/s
Average session durationminimum 0.1 s
60 s
Session table sizeminimum 1 entries
1,000,000 entries
Growth headroomaccepts 0–300 %
25 %

Results

Concurrent Sessions
120,000
Planned Sessions With Growth
150,000
Session Table Utilisation
15.0%
Spare Table Entries
850,000
Max Sustainable New-Session Rate
16,667 sessions/s

Step by step

  1. 1Values used
  2. 2Session Capacity
  3. 3Rate the table can sustain
  4. 4Concurrent Sessions= 120,000
  5. 5Planned Sessions With Growth= 150,000
  6. 6Session Table Utilisation= 15.0
  7. 7Spare Table Entries= 850,000
  8. 8Max Sustainable New-Session Rate= 16,667 sessions/s

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 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

How is Session Capacity calculated?

concurrent sessions = new sessions per second × average session duration (Little's law). Little's law says the population of a stable system equals arrival rate times residence time, so concurrent sessions follow directly from the new-session rate and how long sessions live. Inverting it gives the arrival rate at which the table fills.

Why does Session Capacity matter?

Firewalls and NAT gateways fail by exhausting the session table long before they run out of throughput, and idle-timeout settings drive residence time far more than user behaviour does.

What values do I need to enter?

This calculator takes 4 inputs: New sessions per second, Average session duration, Session table size, Growth headroom. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Related Calculators

Session Capacity Calculator – Calculate Concurrent Sessions