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Why does every SEO tool give a different search volume for the same keyword?
Summary
Because none of them is actually counting searches. Search volume is a model built on Google data and other sources, keyword difficulty is each vendor's own score, and competitor traffic is an estimate stacked on another estimate. The closest you get to a measured number is the clicks and impressions Search Console reports for your own site.
Two vendors, three metrics
| Metric | Ahrefs | Semrush |
|---|---|---|
| Search volume | Keyword Planner as the base, refined with Google Trends and third-party data; 12-month average | Third-party data overlaid with historical clickstream data and run through machine-learning models; 12-month average, updated monthly |
| Keyword difficulty | Weighted average of linking domains to the top 10 pages, on a logarithmic 0–100 scale; nothing else counted | Median referring domains, median dofollow/nofollow ratio and median Authority Score of the ranking domains, plus SERP qualities; 0–100 in six bands |
| Organic traffic | Every keyword ranking in the top 100, estimated from position, monthly volume and Ahrefs' own CTR model, then summed | Based on the domain's organic positions in the top 100 |
Search volume
Ahrefs is open about the weak spot in its base layer. Keyword Planner lumps keywords with similar meanings together and rounds volumes into buckets, so Ahrefs pulls in other sources to split those clusters back into individual terms. Semrush gets there through clickstream data instead. Both land on a 12-month average from different ingredients, and Ahrefs says outright that search volume in any SEO tool is an estimate and only Google has the exact data.
Keyword difficulty
Ahrefs' own blog notes that the major tools share a 0 to 100 scale but each calculates it differently, so the numbers can be far apart. A 40 in Ahrefs describes backlinks and nothing more. A 40 in Semrush also folds in domain authority and what the results page looks like. Setting the two side by side compares different measurements.
Competitor traffic
Neither vendor can see a competitor's analytics. Ahrefs builds its traffic figure from an estimated volume and an estimated click-through rate, and says its CTR figures are approximations from its own model. Semrush's separate traffic tools use another source altogether, a clickstream panel it describes as more than 200 million anonymized users.
Backlinks
Each vendor counts the links its own crawler has found. Ahrefs says its link index is refreshed every 15 to 30 minutes. Semrush says its crawler covers about 10 billion pages a day and its database holds more than 43 trillion backlinks. Two crawlers reach different parts of the web, so the counts for one domain won't line up.
Search Console has gaps too
To protect privacy, the Performance report leaves queries that are searched only a handful of times out of its table, though they still count toward the chart totals. The table stops at 1,000 rows, and new data usually shows up after two to three days.
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Sources
- https://ahrefs.com/academy/how-to-use-ahrefs/ahrefs-seo-metrics/keyword-and-search-traffic
- https://help.ahrefs.com/en/articles/72571-how-accurate-is-keyword-search-volume-in-ahrefs
- https://ahrefs.com/blog/keyword-difficulty/
- https://ahrefs.com/big-data
- https://www.semrush.com/kb/683-what-is-search-volume-in-semrush
- https://www.semrush.com/kb/1158-what-is-kd
- https://www.semrush.com/kb/997-semrush-data
- https://www.semrush.com/kb/494-organic-rankings-positions-report
- https://support.google.com/webmasters/answer/17010575?hl=en
Figures come from each vendor's own pricing pages and documentation. We have not tested these tools hands-on, and no ranking here is paid for.