What is MOS?

MOS, Mean Opinion Score, is the single number from 1 to 5 that tries to summarise how good a call sounds. Here is where that number actually comes from.

A single number for call quality

MOS stands for Mean Opinion Score, a single number from 1 to 5 meant to represent how good a call actually sounds. A score above 4.0 is considered excellent, close to a landline. Below 3.5 and most listeners start noticing real degradation.

It started with real humans

The original MOS methodology was genuinely simple: play a sample of degraded audio to a panel of human listeners and ask them to rate it from 1 to 5, then average their scores. This produced trustworthy results but was completely impractical for monitoring a live network in real time.

Now it is calculated, not asked

Modern systems estimate MOS using a formula instead of a listening panel, most commonly something called the E-model. It takes the measurable numbers, latency, jitter, and packet loss, along with which codec is in use, and produces an estimated score without a single human ever listening.

This is what almost every modern monitoring dashboard actually shows you: a calculated approximation standing in for what a room full of people would have rated the call, decades ago.