SEO ROI calculation, done properly, requires four inputs and one calculation. Inputs: (1) Total SEO investment over the measurement period — agency retainer + internal team time + tooling ($Ahrefs/Semrush) + content production. (2) Organic-attributed revenue — for e-commerce, last-click revenue from Organic Search (GA4); for service businesses, qualified leads from organic landing pages multiplied by close rate and average deal value. (3) Assisted conversions — organic visits that did not last-click convert but appeared in the conversion path, weighted at 25–50% of last-click value depending on attribution model. (4) Brand-search lift — incremental branded query volume attributable to organic-driven brand exposure, valued at the per-click cost you would otherwise pay for branded PPC. ROI calculation: (organic-attributed revenue + assisted conversions value + brand-search lift value − total SEO investment) ÷ total SEO investment. For most Canadian SMB engagements running 12+ months, ROI calculated this way runs 2–6× — significantly higher than the 0.5–1.5× most agencies report by counting only last-click organic revenue.