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Session Table Size Calculator

Work out session table size instantly with clear inputs, formula shown and shareable results.

About Session Table Size Calculator

Session table occupancy follows Little's law using the full lifetime, which includes the idle timeout — a session that finished 90 seconds ago still holds its entry until the timeout expires. With a 300-second idle timeout and 90-second sessions, three quarters of the table is dead weight, which is why timeout tuning is the cheapest way to reclaim firewall capacity.

Formula

Session table

entries = new sessions per second x (duration + idle timeout); active = rate x duration; memory = entries x bytes per entry

Worked example

With the default values below, Session Table Size Calculator returns Session table entries required of 1,560,000. Change any input above and every figure here is recalculated in your browser.

Inputs used

New sessions per secondminimum 0.001
4,000
Average session durationminimum 0.001 s
90 s
Idle timeoutminimum 1 s
300 s
Memory per session entryminimum 1 bytes
320 bytes

Results

Session table entries required
1,560,000
Genuinely active sessions
360,000
Memory for the session table
476.07 MiB
Share of entries that are idle but not yet expired
76.9%

Step by step

  1. 1Values used
  2. 2Session table
  3. 3Session table entries required= 1,560,000
  4. 4Genuinely active sessions= 360,000
  5. 5Memory for the session table= 499,200,000
  6. 6Share of entries that are idle but not yet expired= 76.9

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

What happens when the session table fills?

New connections are refused or existing sessions are evicted, which looks to users like random connection failures. It is also the mechanism behind state-exhaustion attacks.

Should the idle timeout just be shortened?

Carefully. Long-lived idle connections such as database pools and SSH sessions break when the state is dropped, so use per-application timeouts rather than one global value.

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