KD scores vary meaningfully between tools (Ahrefs, Semrush, and others each use their own formula and their own backlink index), so a KD score should never be compared across tools as if it were a standardized measurement — a keyword scored 'easy' in one tool might show 'medium' in another. Most KD formulas weight backlink authority of the current top-ranking pages heavily, which means they can understate difficulty for topics where content quality and search intent match matter more than raw link authority, and understate difficulty on SERPs dominated by very large sites with generically strong (but not topically focused) authority. KD is a useful triage filter for prioritizing a long keyword list, not a precise forecast — actual ranking difficulty also depends on a site's own existing authority and topical relevance, which no generic KD score accounts for.
Example
A keyword scored 'KD 35 - Medium' still takes six months to crack page one for a brand-new site with no existing topical authority, while an established competitor with strong topical relevance ranks for it within weeks.
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