AI share of voice
AI share of voice is the fraction of all brand mentions, across many sampled answers to a fixed set of category questions, that belong to one brand. If the assistants named brands 400 times across a week of runs and 100 of those were yours, your share of voice is 25 percent.
The formula
Your mentions divided by all brand mentions across the sampled answers, computed per engine and overall. It puts you and your competitors on the same scale, which is why it is the headline metric and why it needs visibility rate next to it.
Where it misleads
It is relative, so it rises when a competitor disappears even if you are named no more often. It swings on small samples, so it needs a sample-size floor. And it depends entirely on the prompt set, so the set must stay fixed while periods are compared.