Keyword Difficulty
Keyword Difficulty (KD) is a 0–100 score that estimates how competitive it is to rank in Google’s top 10 organic results for a specific keyword. A KD of 0 indicates almost no competition; a KD of 100 indicates that the top results are dominated by the most authoritative sites on the web.
Example
Semrush reports the keyword “best seo service” at a KD of approximately 92 (very hard). Ahrefs reports the same keyword at approximately 76 (hard). The discrepancy is not a measurement error — it reflects different methodologies. Both are telling you the same thing: this is a hard keyword for a new site to rank for.
Meanwhile, a keyword like “seo tool for wordpress under 100 dollars” might score KD 15 on Semrush and KD 12 on Ahrefs — both indicating a relatively achievable ranking opportunity.
How KD is Calculated
Each tool uses a different formula, but most weight:
- Domain authority of top-10 results — if all 10 results are from high-DA domains, the keyword is harder
- Number of backlinks to ranking pages — more linking domains = higher difficulty
- Content quality signals — some tools factor in whether the top results are comprehensive, long-form resources
- SERP feature presence — featured snippets and knowledge panels can reduce click-through even if you rank
Semrush’s KD specifically weights the “authority score” of the ranking domains (a composite metric that includes organic traffic quality, not just link count). Ahrefs’ KD weights the number of referring domains to the top-10 pages.
Why KD Estimates Differ Between Tools
The same keyword showing KD 92 in Semrush and KD 76 in Ahrefs is normal. Neither number is wrong. The tools are measuring different (but correlated) signals. What matters for your strategy:
- Both tools agree directionally: this is a hard keyword
- Use the same tool consistently within one project so your difficulty comparisons are apples-to-apples
- Don’t compare KD scores across different tools without noting which tool generated them
The Gate-20 Calculation Nobody Does
Every SEO tool shows you keyword difficulty. None of them tell you what it will actually cost to rank.
The calculation that matters: KD score x domain authority gap x content production cost.
Specifically: multiply the keyword difficulty score by the gap between your domain authority and the average DA of the top-3 ranking pages. Then estimate the content production cost (your time or a writer’s fee) to produce a page that competes. That total is your “ranking cost” — a figure that is almost never shown in keyword tools but is the only one that lets you prioritise by ROI.
Read how to do this calculation.
Tools That Show KD
- Semrush Pro at $139.95/mo — Keyword Magic Tool shows KD alongside search volume, CPC, SERP features, and intent. The most granular keyword filtering of any tool.
- Ahrefs Lite at $129/mo — Keywords Explorer shows KD with a clear methodology explanation and the number of backlinks needed to rank in the top 10 (a uniquely transparent metric).
- Moz Pro at $99/mo — Keyword Explorer shows KD with SERP overlay showing which page types dominate each result.