Domain authority isn't a single official metric—it's a third-party score that predicts how well a site might rank. The three main tools are Moz (Domain Authority 0–100), Ahrefs (Domain Rating 0–100), and Semrush (Authority Score 0–100). Each uses its own link index and algorithm, so scores differ between platforms. Moz DA weighs linking root domains and link quality. Ahrefs DR focuses heavily on the number and DR of referring domains using a logarithmic scale. Semrush blends backlink data with organic search performance. To check DA, create a free or paid account with any tool, enter the domain, and view the score on the overview dashboard. Moz offers a limited free browser extension. Ahrefs and Semrush require paid plans for full access, typically starting around 100–130 USD per month. At Ottawa SEO, we primarily use Ahrefs DR because its index updates faster and correlates well with actual ranking ability in competitive Canadian markets like Toronto legal or Ottawa tech. We treat DA/DR as rough directional signals, not absolutes—a site with DR 45 and topically relevant links often outranks a DR 60 site with spammy backlinks. When evaluating link prospects or acquisition targets in our portfolio, we combine DR with organic traffic estimates, referring domain growth trends, and manual spam checks. A domain that jumps 20 DR points in two months is a red flag for PBN activity. Similarly, a site losing DR while gaining traffic usually means it's cleaning up toxic links, which is healthy. Don't obsess over your own DA day-to-day—it updates monthly at best and fluctuates as competitors gain links. Focus instead on acquiring high-quality, editorially earned backlinks from relevant sources. DA will follow if you do the work right.