Estimate bandwidth delay product, TCP throughput, or required TCP window size from link bandwidth, RTT, and path distance for TCP links.
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Bandwidth Delay Product Formula
The calculator uses one of three formulas depending on the mode you pick.
BDP from bandwidth and RTT:
BDP (bits) = Bandwidth (bits/s) × RTT (s)
Throughput from TCP window and RTT:
Throughput (bits/s) = Window (bytes) × 8 / RTT (s)
BDP from distance:
RTT ≈ 2 × Distance / Propagation Speed BDP = Bandwidth × RTT
- BDP — bandwidth-delay product, the data in flight on the path
- Bandwidth — link capacity in bits per second
- RTT — round-trip time in seconds
- Window — TCP send or receive buffer size in bytes
- Distance — one-way path length
- Propagation speed — about 200,000 km/s in fiber, 299,792 km/s in free space
The distance mode gives a propagation-only estimate. Real RTT also includes queuing, serialization, and routing delay, so measured pings are usually higher than the calculated value. Divide BDP bits by 8 to get bytes, the size a TCP window must reach to fill the link.
Reference Values
Typical RTTs by path type:
| Path | Typical RTT |
|---|---|
| Same-rack LAN | 0.1 to 0.5 ms |
| Same-city ISP | 5 to 15 ms |
| Cross-country (US) | 60 to 90 ms |
| Transatlantic | 70 to 120 ms |
| Transpacific | 120 to 180 ms |
| Geostationary satellite | 500 to 700 ms |
BDP for common bandwidth and RTT pairs:
| Bandwidth | RTT | BDP |
|---|---|---|
| 100 Mbps | 40 ms | 500 KB |
| 1 Gbps | 40 ms | 5 MB |
| 1 Gbps | 100 ms | 12.5 MB |
| 10 Gbps | 80 ms | 100 MB |
| 10 Gbps | 600 ms | 750 MB |
Worked Example and Notes
Example. A 1 Gbps link with 80 ms RTT.
BDP = 1,000,000,000 × 0.080 = 80,000,000 bits = 10,000,000 bytes ≈ 10 MB.
The default 64 KB TCP window would cap a single flow at 64,000 × 8 / 0.080 = 6.4 Mbps, well under 1% of the link. To fill the pipe, enable window scaling and set send and receive buffers to at least 10 MB on both ends.
Why the 64 KB limit exists. The TCP header reserves 16 bits for the window field, giving a maximum of 65,535 bytes without the window scale option (RFC 7323). Modern stacks negotiate scaling automatically, but the OS buffer ceiling can still cap throughput.
Distance mode caveat. The calculator assumes a straight-line path. Real fiber routes are longer than great-circle distance, and routers add queuing and processing delay. Treat the distance-mode result as a lower bound on RTT.
