TCP Window Size Calculator
Size the TCP receive window for a given bandwidth and RTT, and find the window scale factor it needs.
Inputs
Required Window Size
2 MiB
Window Scale Factor (shift)
6
Window Available With That Scale
4 MiB
Throughput Ceiling Without Scaling
13.11Mbit/s
Segments In Flight
1,712
Step by step
Values used
Link bandwidth = 500 Mbit/s; Round-trip time = 40 ms; Maximum segment size = 1,460 bytes
TCP Window Size
window = bandwidth × RTT ÷ 8, and the scale factor is ceil(log2(window ÷ 65535)).
Throughput ceiling
throughput ceiling = window ÷ RTT
Required Window Size
= 2,500,000
Window Scale Factor (shift)
= 6
Window Available With That Scale
= 4,194,240
Throughput Ceiling Without Scaling
= 13.11 Mbit/s
Segments In Flight
= 1,712
How it works
A sender may have at most one window of unacknowledged data outstanding, so the window has to cover everything the pipe holds for one round trip. The 16-bit window field stops at 65,535 bytes, so anything larger needs the RFC 7323 window scale option, which multiplies the advertised value by 2^shift. On any long fat pipe the window, not the link, sets the ceiling: a 64 KB window over 40 ms RTT tops out near 13 Mbit/s no matter how much bandwidth you bought.
Formulas
TCP Window Size
window = bandwidth × RTT ÷ 8, and the scale factor is ceil(log2(window ÷ 65535)).
- 65535
- Largest window the 16-bit header field can express
- window
- Receive window in bytes
- RTT
- Round-trip time in seconds
Throughput ceiling
throughput ceiling = window ÷ RTT
- window
- Advertised receive window in bits
- RTT
- Round-trip time in seconds
Frequently Asked Questions
How is TCP Window Size calculated?
window = bandwidth × RTT ÷ 8, and the scale factor is ceil(log2(window ÷ 65535)). A sender may have at most one window of unacknowledged data outstanding, so the window has to cover everything the pipe holds for one round trip. The 16-bit window field stops at 65,535 bytes, so anything larger needs the RFC 7323 window scale option, which multiplies the advertised value by 2^shift.
Why does TCP Window Size matter?
On any long fat pipe the window, not the link, sets the ceiling: a 64 KB window over 40 ms RTT tops out near 13 Mbit/s no matter how much bandwidth you bought.
What values do I need to enter?
This calculator takes 3 inputs: Link bandwidth, Round-trip time, Maximum segment size. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Why does a fast link still transfer slowly?
Because throughput is capped at window ÷ RTT. If the receive window or the socket buffer is small, a single TCP stream cannot fill the link regardless of its capacity — the fix is window scaling and larger buffers, not more bandwidth.