Advanced TCP Window Size Calculator

Calculate receive windows, scaling shifts, and advertised values. Visualize throughput limits from bandwidth-delay product curves. Tune connections confidently across WAN, cloud, campus, and environments.

Use bandwidth, RTT, efficiency, scaling, and buffer assumptions to estimate an engineering-ready receive window and expected path throughput.

Calculator Inputs

Formula Used

Effective Bandwidth = Raw Bandwidth × Utilization × (1 − Overhead) × Application Efficiency

Bandwidth-Delay Product = Effective Bandwidth × RTT / 8

Per-Stream BDP = Bandwidth-Delay Product / Parallel Streams

Recommended Window = Per-Stream BDP × (1 + Safety Margin)

Advertised Window Field = ceil(Window / 2^Scale Shift)

Estimated Throughput = min(Effective Bandwidth, Advertised Window × 8 / RTT)

The calculator also reports the classic no-scaling limit using 65,535 bytes, which is useful when comparing legacy behavior against scaled receive windows.

How to Use This Calculator

  1. Enter the path bandwidth and choose its unit.
  2. Provide the average round trip time for the flow.
  3. Set MSS, expected utilization, overhead, and application efficiency.
  4. Choose a safety margin to cover burstiness and tuning headroom.
  5. Enter the receive buffer limit and number of parallel streams.
  6. Use auto scaling for a recommended shift, or test a manual value.
  7. Submit the form to view the result above the calculator.
  8. Review the graph, then export the summary using CSV or PDF.

Example Data Table

Parameter Example Value Calculated Output
Bandwidth / RTT 500 Mbps / 80 ms BDP = 4.21 MB
Utilization / Overhead / App Efficiency 92% / 2% / 98% Effective Bandwidth = 441.78 Mbps
Safety Margin / Streams 20% / 1 Recommended Window = 5.06 MB
Receive Buffer / Auto Scaling 32 MB / Auto Scale Shift = 7
Advertised Field / Actual Window 41418 / Scaled Active Window = 5.06 MB
MSS / Estimated Throughput 1460 bytes 3,631.17 segments, 441.78 Mbps

FAQs

1) What is TCP window size?

TCP window size is the amount of unacknowledged data a receiver allows in flight. Larger windows help long-latency, high-bandwidth paths reach higher throughput.

2) Why is bandwidth-delay product important?

The bandwidth-delay product estimates bytes needed in transit to fully utilize a path. It connects link rate and round-trip latency directly.

3) What does window scaling do?

Window scaling extends the classic 65,535-byte TCP window. It shifts the advertised field, allowing much larger effective receive windows on modern networks.

4) Does MSS change the window size?

MSS does not set the window directly, but it affects segments in flight. The calculator uses MSS to estimate packet counts inside the chosen window.

5) What happens when the receive buffer is too small?

A buffer cap smaller than the recommended window limits throughput. In that case, increase socket buffers or accept a lower achievable transfer rate.

6) Can I use this for multiple TCP streams?

Yes. Dividing by parallel streams estimates the per-stream window when traffic is spread across multiple simultaneous TCP connections.

7) Does this guarantee real throughput?

No. It provides engineering estimates using input assumptions. Real throughput also depends on congestion control, packet loss, receiver behavior, and operating system tuning.

8) When should I use manual scaling?

Use auto scaling for most cases. Manual scaling is useful when validating device settings, lab captures, legacy stacks, or documented configuration standards.


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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.