Packets, and what happens when one goes missing
Data does not cross the internet as one continuous stream. It is cut into packets, typically up to about 1,500 bytes each, and every packet makes its own way from sender to receiver through a chain of routers. Packet loss is the percentage of those packets that set out and never arrive. A router with a full queue drops the newest arrivals; a wifi frame corrupted by interference fails its checksum and is thrown away; a faulty cable scrambles a few bits and the packet is discarded.
What happens next depends on the protocol. Web pages, downloads and streaming video mostly travel over TCP, which notices the gap and asks for the missing packet again. You never see the loss, but you pay for it in delay, because everything after the gap waits for the resend. Games, voice calls and video meetings mostly use UDP, which never resends. For them a lost packet is simply gone, and the app has to cover the hole: a clipped syllable, a frozen frame, a character that jumps.
What packet loss looks like in practice
| Activity | Symptom of packet loss |
|---|---|
| Online games | Rubber-banding, shots that do not register, opponents teleporting, disconnects |
| Voice calls | Robotic or metallic audio, clipped words, short silences |
| Video calls | Frozen picture, blocky video, audio cutting out, "your connection is unstable" |
| Streaming video | Buffering, drops to lower resolution, usually only at higher loss |
| Browsing and downloads | Pages that stall and then load, slower downloads than the line should manage |
How much packet loss is normal
On a healthy wired connection, packet loss to a nearby server is usually zero or a fraction of a percent. Anything up to about 0.5% goes unnoticed in practice. Between 1% and 2.5% real-time apps start to show it, and above 5% most of them struggle badly, even though downloads and streaming keep working thanks to TCP's resends. There is a longer table by activity in how much packet loss is acceptable.
The pattern matters as much as the number. Two percent spread evenly through a minute is a steady annoyance. Two percent arriving as one burst of half a second is a single frozen moment. That is why the packet loss test on this site draws every packet on a timeline instead of printing only the percentage.
Where packet loss comes from
Most loss that a home user can do anything about happens close to home: weak or crowded wifi, a router that is overwhelmed, an upload that fills the line, or a damaged cable. Further out, a provider's congested link at peak time, a faulty piece of equipment at the exchange, or a badly performing route to one particular destination can all drop packets. What causes packet loss goes through each of these and how to tell them apart.
How to check for packet loss
Run a test that uses UDP, runs for at least 30 seconds and sends packets at a realistic rate. A quick speed test is no use, because it runs over TCP and the resends hide the loss completely. The command-line ping tool works in a pinch but sends one packet a second, which can miss short bursts; how to check packet loss covers both the browser test and the Windows, Mac and Linux commands.