Share of Model (SoM) is the percentage of all brand mentions in AI-generated answers that belong to your brand, measured over a fixed set of category prompts and AI engines. If ChatGPT, Gemini, and Perplexity produce 5,000 brand mentions while answering 1,000 buying-intent prompts about your category, and 1,200 of those mentions name you, your Share of Model is 24%. It is the market-share metric of generative engine optimization (GEO): share of voice measured advertising weight, share of search measured demand, and SoM measures how often AI models actually put your brand in the answer.
Key takeaways
- The formula: SoM = your brand's mentions in AI answers ÷ total brand mentions in those answers × 100, always computed over a fixed prompt set, engine mix, and time window.
- SoM is relative. It tells you how much of the AI conversation you own versus competitors, not whether you appear at all — that is mention rate, a separate AI visibility metric.
- Raw mention counts hide three things that change outcomes: position in the answer, sentiment, and whether the engine cites your pages.
- LLM outputs vary between runs, so a single measurement is noise. Reliable SoM comes from re-running the same prompts on a schedule and reading the trend.
- You grow SoM the way you grow any GEO outcome: unambiguous entity signals, citations in sources the models trust, and content shaped like answers.
From share of voice to Share of Model
Every era of marketing had a dominance metric. Mass media had share of voice: your fraction of category advertising. The Google era added share of search: your fraction of category queries, a workable proxy for demand. Share of Model is the successor for the AI era, and it answers a different question — not how loud you are, and not how many people look for you, but how often the models recommend you when no results page is involved.
The shift matters because the funnel moved. Gartner predicted that traditional search volume would drop 25% by 2026 as users switch to AI assistants, and buying research now routinely ends inside the answer itself — the dynamic behind zero-click search. When a shopper asks for the best mineral sunscreen for sensitive skin and acts on the three-brand shortlist that comes back, the shortlist is the market. SoM is your slice of it.
