TCP Throughput Calculator | BDP & Window Size

TCP throughput calculator: enter bandwidth, RTT and window size to get the bandwidth-delay product, max throughput, loss limit and Linux sysctl tuning. Free.

Also known as: tcp throughput calculator · bandwidth delay product calculator · bdp calculator · tcp window size calculator · tcp window scaling calculator · mathis formula calculator · max tcp throughput latency · tcp buffer size tuning · rwin calculator

network tools

devops and infrastructure tools

networking

Path and Connection

Link bandwidth

The slowest hop on the path, not your NIC speed.

Round-trip time (ms)

From ping or mtr under load. New York to Chicago is about 20 ms, to Los Angeles about 70 ms.

TCP window or socket buffer

64 KiB is the ceiling without window scaling. The Linux autotuning maximum is 6 MiB receive, 4 MiB send.

Packet loss % (optional)

Path MTU

IP version

TCP timestamps

Throughput Result

The bandwidth-delay product, the window and loss limits, and Linux buffer settings will appear here.

About TCP Throughput Calculator

A TCP throughput calculator is used to find out why a fast link moves data slowly over distance. Enter the link bandwidth, the round-trip time and the TCP window, plus packet loss if you know it, and it returns the bandwidth-delay product, the throughput one connection can reach, which limit is holding it back, the window scale factor the path needs, and Linux buffer settings to fix it.

Key Features

Computes the bandwidth-delay product in bytes, KiB and MiB from bandwidth and RTT
Shows window-limited throughput, so you can see a 64 KiB window cap a 1 Gbps link at about 10 Mbps across 50 ms
Adds a loss-limited ceiling from the Mathis formula and the highest loss rate one stream can survive at full rate
Works out the RFC 7323 window scale shift and flags paths that need more than the 1 GiB maximum window
Derives MSS from the path MTU for IPv4 or IPv6, with or without the 12-byte TCP timestamp option
Generates net.core and net.ipv4.tcp_rmem / tcp_wmem sysctl lines sized to twice the BDP, plus an iperf3 test command

Use Cases

Explain why an S3 upload from a us-east-1 instance to a client in Los Angeles stalls far below the instance's network cap
Size socket buffers for a database replica streaming WAL across the country
Decide whether rsync over a 10 Gbps link needs tuning or parallel streams
Check whether 0.1% loss on a VPN path, not bandwidth, is what caps a backup job
Work out how many parallel streams a transfer tool needs to fill a long fat network
Answer a ticket asking why iperf3 shows 1 Gbps on the LAN but 90 Mbps to a remote office

Supported Formats

Bandwidth in Kbps, Mbps or Gbps

Window in bytes, KiB or MiB

IPv4 and IPv6 MTU and MSS, TCP timestamps on or off

Related Keywords

tcp throughput calculator

bandwidth delay product calculator

bdp calculator

tcp window size calculator

tcp window scaling calculator

mathis formula calculator

max tcp throughput latency

tcp buffer size tuning

rwin calculator

Why is my transfer slow on a fast link with high latency?

Because TCP can only have one window of unacknowledged data in flight per round trip. Throughput is capped at window / RTT, no matter how fast the link is. A 64 KiB window over a 50 ms round trip moves at most 65,536 x 8 / 0.05 = about 10.5 Mbps, even on a 10 Gbps link. To fill the link, the window has to be at least the bandwidth-delay product: bandwidth x RTT / 8, which is 6.25 MB for 1 Gbps at 50 ms. Modern stacks negotiate window scaling and autotune their buffers, but the autotuning ceiling (6 MiB receive and 4 MiB send by default on Linux) is often smaller than the BDP of a fast cross-country path, and any packet loss lowers the ceiling further.

Calculated in your browser

All arithmetic runs locally in JavaScript. Nothing about your network paths or hosts is sent anywhere.

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Frequently asked questions

What is TCP Throughput Calculator used for?

TCP throughput calculator: enter bandwidth, RTT and window size to get the bandwidth-delay product, max throughput, loss limit and Linux sysctl tuning. Free.

Is TCP Throughput Calculator free to use?

Yes. TCP Throughput Calculator is completely free with no usage limits, and there is no paid tier required to use it.

Do I need an account to use TCP Throughput Calculator?

No. TCP Throughput Calculator works immediately in your browser with no signup, login, or email address required.

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