TCP Throughput & Bandwidth-Delay Product Calculator
Work out the bandwidth-delay product, the TCP window you need, window- and loss-limited throughput (Mathis formula), header overhead and transfer time for a link.
The window, not the link, sets the speed: raise the receive window to at least 625,000 bytes (modern systems auto-tune it; check that window scaling is not disabled by a firewall or old middlebox).
Overhead assumes plain IPv4 and TCP headers (40 bytes); TCP timestamps add 12 bytes and IPv6 another 20, so goodput is slightly lower on such paths.
| RTT (ms) | Throughput (Mbit/s) | BDP (bytes) |
|---|---|---|
| 1 | 94.9285 | 12,500 |
| 5 | 94.9285 | 62,500 |
| 10 | 52.428 | 125,000 |
| 20 | 26.214 | 250,000 |
| 50 | 10.4856 | 625,000 |
| 100 | 5.2428 | 1,250,000 |
| 150 | 3.4952 | 1,875,000 |
| 200 | 2.6214 | 2,500,000 |
| 300 | 1.7476 | 3,750,000 |
A fast link does not guarantee a fast download. TCP can only have one receive window of unacknowledged data in flight, so over a long round trip a small window caps the speed no matter how much bandwidth there is, and packet loss caps it further. This calculator works out the bandwidth-delay product (how much data must be in flight to fill the link), the throughput your window allows, the loss limit from the Mathis formula, header overhead and how long a transfer will take.
How to use this tool
- Enter the link bandwidth and the round-trip time (ping) between the two ends.
- Enter the TCP receive window; 65,535 bytes is the maximum without window scaling.
- Optionally add packet loss and change the MSS.
- Read the expected throughput and what limits it, then the window you would need for full speed.
Formula
BDP = bandwidth × RTT ÷ 8; window limit = window × 8 ÷ RTT; Mathis limit = (MSS ÷ RTT) × √(3/2) ÷ √p; throughput = min(link goodput, window limit, Mathis limit)
Bandwidth is in bit/s, RTT in seconds, window and MSS in bytes, p the packet-loss probability. Link goodput on Ethernet = bandwidth × MSS ÷ (MSS + 78), where 78 bytes are the IP and TCP headers (40) plus Ethernet framing, preamble and inter-frame gap (38). A window larger than 65,535 bytes needs the window-scale option of RFC 7323.
Example
On a 100 Mbit/s link with a 50 ms round trip, BDP = 100,000,000 × 0.05 ÷ 8 = 625,000 bytes. A classic 65,535-byte window allows only 65,535 × 8 ÷ 0.05 = 10.49 Mbit/s, about a tenth of the link, and 1 GB takes 12 min 43 s. With a 1 MiB window the link itself becomes the limit at about 94.9 Mbit/s of data.
Frequently asked questions
Why is my download slow on a fast connection far away?
Distance raises the RTT, and throughput per TCP connection is at most window ÷ RTT. If the window is too small, or loss forces TCP to back off, a single connection cannot fill the link. Download tools that open several connections work around this.
What is window scaling?
The TCP header has only 16 bits for the window, so the largest window is 65,535 bytes. The window-scale option (RFC 7323) multiplies it by up to 2¹⁴. Modern operating systems use it and tune the window automatically, but some firewalls strip the option.
Why can I never reach 100 Mbit/s on a 100 Mbit/s link?
Headers take space. With a 1500-byte MTU each frame carries 1,460 bytes of data in 1,538 bytes on the wire, so the best possible TCP goodput is about 94.9% of the link rate.
Sources
Last updated:
Page address with these inputs: https://kakao.mobile/tools/throughput-tool/
Cite or embed this tool
kakaoCalc. (2026, November 8). TCP Throughput & Bandwidth-Delay Product Calculator. https://kakao.mobile/tools/throughput-tool/
“TCP Throughput & Bandwidth-Delay Product Calculator.” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/throughput-tool/.
Paste this code into your page. The widget runs in your visitors' browsers, shows no ads and keeps a link back to this page. Embedding terms