Bandwidth-Delay Product
Bytes in flight on a link from bandwidth and round-trip time, the TCP window that fills it, and the throughput one stream reaches with the window you have.
The bandwidth-delay product in bytes and KiB, the window a single TCP connection needs, the throughput it gets with a given window, and the 64 KiB ceiling without window scaling.
Example: 1 Gbps at 80 ms RTT holds 10 MB in flight; a 4 MiB window caps one stream at 419 Mbps, and without window scaling (64 KiB) at 6.55 Mbps.
Capacity times latency,
in flight.
Why a fast link across a long distance is slow for one connection, what the window has to do with it, and the numbers behind window scaling.
Bytes in flight
Data sent but not yet acknowledged sits in the pipe. To keep a link busy the sender must have at least bandwidth × round-trip time outstanding — the bandwidth-delay product. 1 Gbps × 80 ms = 80 Mbit = 10 MB. The RTT is the round trip because acknowledgements have to come back before the window opens again.
The window limit
A TCP sender may not have more than one window unacknowledged, so one stream cannot exceed window ÷ RTT whatever the link speed. The original 16-bit window field caps at 65,535 bytes: at 80 ms that is 6.55 Mbps, which is why window scaling (RFC 7323) exists. The page reports the throughput your window allows and the window needed to fill the link; the achievable rate is the smaller of that and the link.
Idealised
One connection, no loss, RTT as measured, and “window” meaning the smaller of the sender’s congestion window and the receiver’s advertised window. Loss-based congestion control needs buffer beyond the BDP to recover without draining the pipe, and operating systems autotune windows up to a configured maximum — that maximum is often the real ceiling.
What the number does not settle
Application-level throughput also depends on the far end, disk speed, encryption and framing; parallel streams sidestep the per-connection cap and are not modelled. Download & Transfer Time gives the link-speed bound without the latency term. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
Last reviewed 18 September 2026. How results are checked: How we verify.