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Keyword density is the percentage of times a target keyword appears relative to the total word count on a page. It matters because it signals to search engines what your page is about. The ideal range is 0.5–3%. Below 0.5% and Google may not associate the page strongly with the keyword. Above 4–5% and your page risks being flagged for keyword stuffing, which can result in a manual or algorithmic penalty.
Keyword stuffing is the practice of overusing a keyword unnaturally in an attempt to manipulate rankings. Google's algorithms penalise pages that repeat keywords excessively. Signs of stuffing include: the same phrase repeated in every paragraph, keywords in unrelated context, hidden text, or keyword lists at the bottom of a page. Write for your reader first, and check density as a diagnostic, not a target.
Check keyword density when you're optimising existing pages for specific keywords, after publishing new content, or when a page drops in rankings unexpectedly. Don't obsess over hitting a specific number — focus on writing naturally and covering the topic comprehensively. Use density as a sanity check: if you've barely mentioned your keyword, add it naturally; if you've used it 20 times in 500 words, thin it out.
Keyword density as a standalone metric is far less important than it was pre-2011. Google's understanding of semantics, entities and intent means it can rank a page for a keyword that barely appears in the text. What matters more today is topical coverage, related terms (LSI keywords), E-E-A-T signals and user engagement. Use density as a rough guide, but focus on comprehensive, expert content over keyword repetition.