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Bandwidth-delay product calculator

Find the TCP window a connection needs to fill a link, or the most a given window allows over a round trip.

Work out

In milliseconds, or with a unit: 0.2 s. Ping shows it.

Adds an estimate of what one connection gets with this much loss.

1460 on most Ethernet links.

Try:

Window needed

1 Gbit/s × 50 ms

Example: type your own bandwidth and round-trip time above.

TCP window needed6,250,000 bytes5.96 MiB, the bandwidth-delay product
Window scaleShift 7×128, up to 8,388,480 bytes
In bits
50,000,000
Most per connection without scaling
10.49 Mbit/s

Both ends must support window scaling, and their buffers must allow a window this big. A 65,535-byte window would cap one connection at 10.49 Mbit/s.

How it works

The bandwidth-delay product (BDP) is how much data is on its way at once when a link is full. Multiply the speed by the round-trip time (RTT, the time for a packet to get there and its reply to come back). At 1 Gbit/s and 50 ms, that's 50,000,000 bits, or 6.25 MB.

TCP sends at most one window of data, then waits for it to be acknowledged. So to fill the link, the receiver's window must be at least the BDP. With a smaller window, one connection can move at most one window per round trip, however fast the link is.

The window field in the TCP header holds at most 65,535 bytes. Window scaling (RFC 7323) multiplies it by 2 to the power of a shift from 0 to 14, agreed when the connection opens. That allows windows up to 1,073,725,440 bytes. Modern systems scale automatically, but buffer limits in the operating system can still cap them.

Packet loss also limits TCP, because each loss makes the sender slow down. To see how long a transfer takes, use the transfer time calculator. For the largest packet a link or tunnel carries, use the MTU calculator.