WDF*IDF: Term Weighting Beyond Keyword Density

Last updated: March 2026 · Reading time: 5 minutes

Keyword density was a central SEO metric for years: "Your keyword must make up 2-3 percent of the text." This was already outdated by 2015. WDF*IDF goes a step further: instead of measuring a term's absolute frequency, the formula puts it in relation to frequency in other documents.

What WDF*IDF Means

WDF (Within Document Frequency) measures how often a term appears in a document — logarithmically weighted to balance extremes.

IDF (Inverse Document Frequency) measures how many documents in the entire index contain the term. Rare terms receive a higher IDF value.

WDFIDF combines both values. A high WDFIDF value means: the term appears frequently in your document but is overall rare. This signals topical relevance.

WDF*IDF in Practice: Benefits and Limits

Useful as an analysis tool. WDF*IDF tools like Termlabs or Ryte show which terms topically relevant documents use that are missing from your text. This helps identify topical gaps.

Not an optimization target. Do not write for a formula. If a WDF*IDF tool says you need the term "crawl budget" more often, check whether that makes topical sense — not whether the curve is right.

Google thinks semantically. Google's ranking algorithm uses neural language models (BERT, MUM) that understand meaning. Term weighting is an outdated concept for Google.

What Replaces WDF*IDF in 2026: Semantic Authority

Instead of counting terms, Google evaluates topical depth, user signals, and authority. AI systems like ChatGPT and Perplexity completely ignore term weighting — they evaluate source authority and clarity of statements.

Write content that comprehensively and understandably covers a topic. This automatically produces a natural term distribution that satisfies any WDF*IDF tool — without you ever looking at the formula.

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What is WDF*IDF?

A term weighting formula that measures how relevant a term is in a document compared to all documents in the index. It replaces simple keyword density with a relative perspective.

Is WDF*IDF still relevant for SEO?

As an analysis tool yes, as a ranking factor no. Google uses semantic models that go far beyond term weighting. WDF*IDF helps find topical gaps — nothing more.

Which tools calculate WDF*IDF?

Termlabs, Ryte, Surfer SEO, and other content optimization tools offer WDF*IDF analyses. They show which terms appear in top-ranking documents and are missing from your text.