Domain Authority (DA) is a search engine ranking score created by Moz, scaled from 0 to 100, with higher scores indicating stronger predicted ranking ability. Moz calculates DA using a machine-learning model trained on their link index, which crawls and analyzes billions of links across the web. The model considers factors like the number of linking root domains, total links, and the quality of those links, then outputs a comparative score. Moz does not publish the exact formula or weighting of inputs because the model evolves as their link data and algorithms improve. Moz recalculates DA as its link index and model change, so a score can fluctuate even when the measured site has not changed. The scale is designed so that moving from 20 to 30 is easier than moving from 70 to 80, because top-tier domains accumulate links at a scale small sites cannot match. Critically, Domain Authority is not a Google ranking factor. Google does not use Moz's score in its algorithm. DA is a third-party metric meant to help marketers compare the relative link strength of domains or track progress over time. It correlates with rankings because both rely on link signals, but improving your DA does not directly improve your Google rankings. You improve DA by earning high-quality backlinks from diverse, authoritative domains—the same activities that help with actual SEO. A site with DA 35 and strong topical authority can outrank a DA 50 site with irrelevant links. Focus on link quality, relevance, and diversity rather than chasing a number that Google never sees. When DA drops, review the provider's index updates and the site's measured link profile rather than assuming a Google penalty or ranking loss.