keyword difficulty

KD is a vendor metric, not a Google one. Google never publishes a difficulty number, so every score you see comes from a tool’s own model of the search results page.

How the tools differ

Ahrefs keeps it simple: it counts the referring domains pointing at the current top 10 and plots that on a logarithmic 0 to 100 scale, so the score is basically a proxy for “how many linking sites would I need.” Semrush uses a weighted formula where median referring domains carry the most weight (around 41%), followed by the median Authority Score of ranking pages, search volume, the follow/nofollow link ratio, and SERP features. Moz builds its score from the Domain Authority and Page Authority of the pages already ranking. Because the inputs differ, the same keyword can read KD 23 in one tool and KD 58 in another. Do not compare scores across tools.

Two limits matter. First, most base KD scores ignore your own site’s authority, so a KD 40 term is a very different bet for a new blog than for an established brand. Second, KD says nothing about search intent or content quality, and a classic KD score predicts the blue-link ranking, not whether you get cited in AI Overviews, ChatGPT, or Perplexity. Use it to filter and sequence a keyword list, then confirm with a manual look at who actually ranks and why before you commit budget.