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As Matt indicated very difficult to compare as it is rarely apple for apples. An exact impression match would not be the same, unless in the same position & it appeared at the same time - which is impossible. It is a rare a paid ad appears every single time a page is searched and certainly not in the same spot. Organic again has its weakness, as the page may not be optimized for search - hence reading its impressions might not be beneficial as being accurate. I am not sure you can choose anything as accurate as such, but often after we have optimized a page we get more impressions for that keyword - than say what semrush states. We use semrush as the target and try not to settle for less than what they state we should achieve. Most of the time it works.   WMT's is now the best way to measure impressions as it has redacted the double up count, where two urls' or more appeared on the same page. What does WMT's state for impressions? In short to answer your question however use keyword planner or semrush as your base.