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Buffer Size Requirement Calculator

Work out buffer size requirement instantly with clear inputs, formula shown and shareable results.

About Buffer Size Requirement Calculator

The classic rule sized router buffers at one bandwidth-delay product so a single TCP flow could keep the link full. The Stanford result showed that with N independent flows, BDP divided by sqrt(N) suffices, because their sawtooths desynchronise. Oversized buffers cause bufferbloat: the link stays full but queueing delay grows to hundreds of milliseconds.

Formula

Buffer sizing

BDP = link rate x RTT / 8; Stanford rule = BDP / sqrt(flows); drain time = buffer bits / link rate

Worked example

With the default values below, Buffer Size Requirement Calculator returns Recommended buffer size of 3.37 MiB. Change any input above and every figure here is recalculated in your browser.

Inputs used

Link speedminimum 0.001 Gbps
10 Gbps
Round-trip timeminimum 0.1 ms
40 ms
Concurrent long-lived flowsminimum 1
200
Sizing ruleone of Classic bandwidth-delay product, Stanford rule: BDP / sqrt(flows), Small buffers: 20 x MTU
Stanford rule: BDP / sqrt(flows)

Results

Recommended buffer size
3.37 MiB
Bandwidth-delay product
47.68 MiB
Time to drain a full buffer
2.83 ms
Latency assessment
Buffer delay is within a reasonable bound

Step by step

  1. 1Values used
  2. 2Buffer sizing
  3. 3Recommended buffer size= 3,535,534
  4. 4Bandwidth-delay product= 50,000,000
  5. 5Time to drain a full buffer= 2.83 ms
  6. 6Latency assessment= Buffer delay is within a reasonable bound

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

What is bufferbloat?

Excessive buffering that keeps a link busy at the cost of latency. Interactive traffic queues behind bulk transfers, so video calls stutter while a download runs.

What fixes it?

Active queue management such as CoDel or FQ-CoDel, which drop or mark packets based on how long they have been queued rather than on queue length.

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