Bandwidth is the most data a connection can carry per second: the 300 Mbps on your plan. Throughput is how much data actually gets through at a given moment, which is what a speed test's download and upload figures measure. Latency is how long a single packet takes to travel there and back, measured in milliseconds. A speed test reports throughput and latency. Your bill describes bandwidth.

The three terms in one table

Term What it means Unit Where you see it
Bandwidth Maximum capacity of a link Mbps or Gbps Your plan, a router's spec sheet, a cable's rating
Throughput Data actually delivered right now Mbps or Gbps Speed test download and upload results, a download window
Latency Delay for one round trip ms Speed test ping, the ping command, in-game ping

In everyday speech, "speed" usually means throughput. ISPs use it to mean bandwidth. That's where much of the confusion starts.

A road analogy, and where it breaks

Think of a highway. Bandwidth is the number of lanes: how many cars could pass per minute if traffic flowed perfectly. Throughput is how many cars actually pass per minute, which is lower when there are roadworks, merging traffic or slow trucks. Latency is how long the trip takes for one car.

The analogy shows the key point: adding lanes doesn't shorten the drive. A gigabit plan doesn't make your ping lower than a 100 Mbps plan on the same network. The data still has to travel the same distance through the same routers.

Where it breaks down: on the internet, the "road" is many links in a row, each with its own capacity. Your throughput is set by the slowest link on the path at that moment. That could be your Wi-Fi, your plan, your ISP's network at peak time, or the server you're downloading from.

Why throughput is lower than bandwidth

A test result a little below your plan is normal. A few things always take a share:

  • Protocol overhead. Every packet carries headers for addressing and error checking, and TCP sends acknowledgements back. On a wired connection that typically costs a few percent. A gigabit Ethernet link, for example, usually tests at a little over 900 Mbps rather than 1,000.
  • The weakest link. Wi-Fi often has less real capacity than the plan, especially through walls. A 100 Mbps Ethernet port or a slow adapter caps everything behind it. See why Ethernet can test slower than Wi-Fi for that case.
  • Sharing. Other devices in your home use the same bandwidth. At busy times, so do your neighbors on the ISP's local network.
  • The other end. A file server or game store can send slower than your connection can receive.

When the gap is large rather than small, read why a speed test can sit below the advertised rate.

How latency limits throughput

Bandwidth and latency are separate, but latency can still hold throughput down. Most internet traffic uses TCP, which sends a batch of data and then waits for confirmation before sending more. The amount it can have "in flight" at once is called the window.

That gives a ceiling for a single connection: window ÷ round-trip time. With a 64 KB window (65,536 bytes) and 100 ms of latency:

65,536 bytes × 8 = 524,288 bits, ÷ 0.1 s = about 5.2 Mbps

Same window, 10 ms latency: about 52 Mbps. So one connection to a distant server can be slow on a fast plan. Modern operating systems grow the window well beyond 64 KB, which raises the ceiling, but distance still matters. That's also why speed tests, SpeedMetric included, run several connections in parallel. Together they fill the link in a way a single connection to a faraway server might not.

Which number matters for what

Different activities stress different measurements:

  • Downloads, game updates, cloud backups: throughput. More is better, and latency hardly matters.
  • Streaming video: steady throughput above the service's requirement. Players buffer ahead, so a few hundred milliseconds of latency won't be noticed.
  • Online games: latency and jitter. Most games use very little bandwidth, but every extra millisecond is felt.
  • Video calls: both, plus upload throughput, since your camera feed goes out over a connection whose upload is often much smaller than its download.

Jitter is a fourth term worth knowing. It measures how much latency varies from one packet to the next. A steady 40 ms ping often feels better than one swinging between 10 and 90 ms.

What a speed test actually tells you

The SpeedMetric speed test measures download and upload throughput, idle ping and jitter, between your browser and a nearby test server. It shows what your connection delivered during those seconds, along the path to that server. It isn't a measure of your plan's bandwidth, and it doesn't show latency under load, packet loss or DNS time.

For a sharper picture of your bandwidth, test on Ethernet at a quiet hour. For a picture of everyday experience, test where and when you actually use the connection. To see latency on its own, a command-line ping test gives you a longer sample. The download, upload and ping guide covers each result in more detail, and if a download window shows MB/s instead of Mbps, convert with Mbps vs MB/s before comparing.