Case 1
- Bandwidth
- 100Mbps
- Round-trip time
- 40ms
4,000.00kbit
Open with these valuesBandwidth-Delay Product Calculator
4,000.00kbit
Result: 4,000.00 kbitMbps counts a million bits per second and a millisecond is a thousandth of a second, so Mbps × ms lands in kilobits of 1,000 bits. A 100 Mbps link with a 40 ms round trip holds 4,000 kbit in flight — 500 kB of TCP window.
The numbers at a glance
Held fixed: Bandwidth 100.00 Mbps.
| Round-trip time (ms) | Result (kbit) |
|---|---|
| 0.0 | 0.00 |
| 10.0 | 1,000.00 |
| 20.0 | 2,000.00 |
| 30.0 | 3,000.00 |
| 40.0Your value | 4,000.00 |
| 50.0 | 5,000.00 |
| 60.0 | 6,000.00 |
| 70.0 | 7,000.00 |
| 80.0 | 8,000.00 |
Worked examples
Case 2
- Bandwidth
- 1000Mbps
- Round-trip time
- 10ms
10,000.00kbit
Open with these valuesCase 3
- Bandwidth
- 50Mbps
- Round-trip time
- 20ms
1,000.00kbit
Open with these valuesHow it's calculated
BDP = bandwidth (Mbps) × RTT (ms)
- StepTake the link bandwidth in megabits per second.
- StepTake the round-trip time in milliseconds, the full loop.
- ResultMultiply the two — the product is the data in flight, in kilobits.
What this number means
This is an amount of data, not a speed
The product says how much sits on the wire at any one moment, not how fast it travels. That volume is what a sender may push out before the first acknowledgement comes back.
Kilobits, not kilobytes
The answer counts kilobits of 1,000 bits each. Divide by 8 for kilobytes: 4,000 kbit is 500 kB.
The two thousands cancel, so just multiply
Mbps is a million bits per second and a millisecond is a thousandth of a second. The two factors of 1,000 cancel, so Mbps × ms lands in kilobits without any conversion.
The return leg counts as well
The input is the round-trip time, not the one-way delay. The window frees up only once the acknowledgement is back.
Commonly misread
A 100 Mbps link with 40 ms carries 4,000 kilobytes in flight.
It carries 4,000 kilobits, which is 500 kB. Bits and bytes differ by a factor of 8 here.
A kilobit here means 1,024 bits, as with memory sizes.
Data rates are decimal, so a kilobit is 1,000 bits. Kibi and mebi are reserved for storage sizes.
A faster link gets by with the same receive window.
The window has to hold the whole product. At 1,000 Mbps and 10 ms that is 10,000 kbit, or 1,250 kB.
Reference table
| Bandwidth, RTT | Typical link | BDP (kbit) |
|---|---|---|
| 1, 1 | Local switch | 1.00 |
| 2.5, 8 | Slow line, short hop | 20.00 |
| 50, 20 | Home fibre, national | 1000.00 |
| 100, 40 | Home fibre, continental | 4000.00 |
| 1000, 10 | Data centre, gigabit | 10000.00 |
Questions
What is the bandwidth-delay product?
It is the amount of data that fits on the wire between two hosts at any moment. Bandwidth times round-trip time gives the volume a sender may push out before the first acknowledgement returns.
Why does the answer come out in kilobits?
Mbps means 1,000,000 bits per second and a millisecond is a thousandth of a second. The two factors of 1,000 cancel, so the product is already in kilobits of 1,000 bits each.
How do I turn it into a TCP window size?
Divide the kilobits by 8 to get kilobytes. 4,000 kbit is 500 kB, and a receive window smaller than that leaves the link idle while it waits.
One-way delay or round-trip time?
Round-trip time. The window only frees up when the acknowledgement comes back, so the return leg counts as much as the outbound one.
Sources and last check
Information, not professional advice.