What this calculator does
Transmission delay, also called serialisation delay, is the time to push a packet onto the wire: packet size in bits divided by the link rate. A 1,500 byte packet on a 100 Mbps link takes 120 microseconds.
Which delay dominates depends on the link. On a short fast link transmission dominates, and on a long link propagation does. A 1,500 byte packet on a 1 Gbps link takes 12 µs to serialise but 24.9 ms to cross 5,000 km, so the distance is almost the whole story on a long haul.
The formula
The packet size is converted from bytes to bits by multiplying by eight, then divided by the link rate in bits per second. Where a distance is given, propagation delay is added to give the total one-way figure. The calculator reports which of the two terms dominates, since that determines what would actually help.
| Term | Meaning |
|---|---|
| Transmission delay | Time to clock the packet onto the link. Falls as bandwidth rises. |
| Serialisation | Another name for the same thing: turning a packet into a serial bit stream. |
| Propagation delay | Time for the signal to travel the distance. Unaffected by bandwidth. |
| MTU | Maximum transmission unit, the largest packet a link carries. 1,500 bytes on standard Ethernet. |
The inputs explained
| Field | What to enter |
|---|---|
| Packet size (bytes) | Packet size in bytes. Standard Ethernet MTU is 1,500. |
| Link rate (Mbps) | Link rate in megabits per second. |
| Link distance (0 to ignore propagation) (km) | Link distance in kilometres, to include propagation. Set to 0 to see transmission alone. |
| Velocity factor | Velocity factor of the medium, about 0.67 for fibre. |
When to use it
Diagnosing a slow link
Knowing which term dominates tells you whether more bandwidth or a shorter path would help.
Sizing packets
Large packets serialise slowly, which matters for latency-sensitive traffic sharing a slow link.
Comparing upgrade options
On a long link, a bandwidth upgrade may barely move total latency.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Which delay dominates?
The same packet and distance at a range of link rates.
| Link rate | Transmission delay | Propagation delay | Total one-way delay |
|---|---|---|---|
| 10 Mbps | 1.200 ms | 0.0498 ms | 1.250 ms |
| 100 Mbps | 0.1200 ms | 0.0498 ms | 0.1698 ms |
| 1,000 Mbps | 0.0120 ms | 0.0498 ms | 0.0618 ms |
Questions
What is the difference between transmission and propagation delay?
Transmission is the time to put the bits onto the wire, set by packet size and bandwidth. Propagation is the time for those bits to travel the distance, set by length and medium. More bandwidth reduces the first and does nothing to the second.
Why does packet size matter for latency?
Because a large packet occupies the link while it serialises, delaying anything behind it. On a slow link a 1,500 byte packet blocks the line for over a millisecond, which is why techniques like link fragmentation exist for latency-sensitive traffic on slow connections.
Does this include queueing?
No. Transmission and propagation are the deterministic components. Queueing delay, which is what varies under load and causes most real-world latency problems, depends on traffic and buffer depth and is not predictable from link parameters alone.
Why convert bytes to bits?
Because link rates are quoted in bits per second while packet sizes are quoted in bytes. The factor of eight between them is a frequent source of errors in back-of-envelope network calculations, so the calculator reports the bit count explicitly.
For the distance component, see the propagation delay calculator. For memory timings, see the RAM latency calculator.