An everyday example
Imagine a friend reading a sentence to you one word per second over a phone. If every word takes exactly half a second to reach you, you hear a perfect sentence, just slightly delayed. If some words take a tenth of a second and others take two seconds, they arrive bunched up, out of rhythm, or too late to make sense. The average delay is the same. The variation is the problem. That variation is jitter.
How jitter is calculated
A speed test measures ping many times in a row and looks at how much each measurement differs from the previous one:
| Sample | Ping | Change from previous |
|---|---|---|
| 1 | 20 ms | — |
| 2 | 22 ms | 2 ms |
| 3 | 21 ms | 1 ms |
| 4 | 35 ms | 14 ms |
| 5 | 20 ms | 15 ms |
The average change is (2 + 1 + 14 + 15) ÷ 4 = 8 ms of jitter. This is the method our test uses on 21 idle ping samples. Real-time apps use a closely related formula defined in RFC 3550, smoothed over the packets of the call itself.
How apps deal with jitter
Call and game software keeps a small jitter buffer: it holds incoming packets for a moment so that late ones can catch up, then plays them out evenly. A bigger buffer copes with more jitter but adds delay. When jitter exceeds what the buffer can absorb, you hear robotic or clipped audio, see frozen video, or feel stutter in a game.
Streaming video uses a much larger buffer, often many seconds, which is why jitter almost never affects Netflix or YouTube once playback has started.
What level of jitter is a problem?
- Under 10 ms: very steady.
- 10–30 ms: normal for many WiFi and mobile connections; most apps cope.
- Over 30 ms: calls and games start to suffer. Microsoft’s Teams guidance targets jitter under 30 ms.
What causes jitter
Jitter comes from anything that makes delay unpredictable:
- WiFi: devices take turns to transmit, retransmit after interference, and change speed as signal varies.
- Busy connections: queues in the router grow and shrink as other traffic comes and goes.
- Mobile networks: changing signal and cell load.
- Congested provider networks, especially at peak times.
Our guide on how to reduce jitter starts with the simplest test: compare WiFi with a cable.