What is the bandwidth-delay product?

The bandwidth-delay product (BDP) is the amount of data that can be in transit on a network path at any given moment. It equals the link bandwidth multiplied by the round-trip time. A TCP window smaller than the BDP will stall the sender while waiting for ACKs, reducing throughput.

Why round to a power of two?

Many TCP implementations (Linux, Windows, BSD) round the advertised receive window to a power of two internally, or apply window scaling in powers of two. Using a power-of-two buffer size avoids waste and aligns with kernel buffer allocation.

How do I apply this on Linux?

Set sysctl net.core.rmem_max and net.ipv4.tcp_rmem (min default max) to at least the recommended value. For example: sysctl -w net.ipv4.tcp_rmem="4096 87380 ". A reboot is not required.

What RTT should I use?

Use the RTT to your most distant or most performance-critical peer. Run "ping " and use the average value. For cross-continental paths expect 80–180 ms; for intercontinental 150–300 ms.

Does TCP window scaling change this?

TCP window scaling (RFC 1323) allows windows larger than 65,535 bytes by using a shift factor. Modern OSes negotiate it automatically. This calculator gives you the raw byte target; your OS handles the scaling negotiation.

TCP Window Size Calculator

Name: TCP Window Size Calculator
Availability: InStock
Author: Nham Vu

Enter your link speed and round-trip time to calculate the optimal TCP window size that maximizes network throughput.

Link Parameters

Link Speed (Bandwidth)

Nominal line rate of your link (e.g. 1000 Mbps = 1 Gbps)

Round-Trip Time (RTT)

Measured ping RTT to the remote host in milliseconds

Current Window Size (optional)

Your current window to compare efficiency against optimal

Quick Presets

Enter link speed and RTT, then click Calculate.

Bandwidth-Delay Product (BDP)

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Bytes

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Recommended Window Size

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Bytes

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Theoretical Max Throughput

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Mbps

—

Gbps

Linux Sysctl Reference

# Set maximum socket receive buffer

net.core.rmem_max = —

net.core.wmem_max = —

# TCP socket buffers: min default max

net.ipv4.tcp_rmem = 4096 87380 —

net.ipv4.tcp_wmem = 4096 87380 —

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Summary

Enter your link speed and round-trip time to calculate the optimal TCP window size that maximizes network throughput.

How it works

Enter your link speed in Mbps or Gbps.
Enter the round-trip time (RTT) in milliseconds.
The calculator computes the bandwidth-delay product: BDP = Bandwidth x RTT.
The BDP is rounded up to the nearest power of two (required by some OS TCP stacks).
Results are shown in bytes, kilobytes, and megabytes for easy reference.
A throughput efficiency estimate shows the impact of a smaller window.

Use cases

Tune Linux tcp_rmem / tcp_wmem sysctl parameters for high-speed WAN links.
Size the TCP receive window for satellite or long-haul fiber connections.
Diagnose slow file transfers where the window is the bottleneck.
Plan buffer sizes for network appliances and load balancers.
Validate iperf3 --window flag values before benchmarking.
Educate network engineers on the BDP concept with real numbers.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu