What Is Domain Authority And Why It's Necessary For Search Ranking

PageRank by Google

The original PageRank algorithm, as described in the seminal 1998 paper by Larry Page and Sergey Brin (“The Anatomy of a Large-Scale Hypertextual Web Search Engine”), is publicly known.

However, the details of how Google uses PageRank today are not publicly disclosed and have evolved significantly since then.

The algorithm treats links as votes, and pages passing their PageRank through outbound links. The mathematical formula and the concept of damping factor, link-based voting, and iterative computation are all well-documented.

The PageRank formula

• PR(A) = PageRank of page A
• d = damping factor (usually 0.85)
• T1 to Tn = pages linking to A
• C(Ti) = number of outbound links from page Ti

Damping factor (d): The likelihood a user continues clicking links (usually set to 0.85).

Inbound link influence: Pages gain rank from links, especially from important and exclusive sources.

The linking pages needed to be dofollow links which pass link equity, while nofollow links didn't contribute to PageRank.

PageRank calculation

PageRank is computed iteratively. All pages start with an initial value and the formula is applied repeatedly until values converge.

It's worth highlighting that PageRank is calculated on a page level rather than domain level, unlike the other metrics mentioned here. Nowadays both page and domain level authority are useful to assess.

Plain explanation

Think of PageRank as voting. A link is a vote, but votes from more important pages count more. Pages that link to many others divide their influence.

Why it's less used directly today

Google retired public PageRank in 2016. It has been replaced or supplemented by algorithms that consider relevance, link context, user behavior, and more.

Still, PageRank remains foundational to understanding modern link-based SEO and all domain-based authority scores.

Domain Authority (DA) by Moz

DA scores domains on a scale from 1 to 100, with higher scores indicating a greater potential to rank.

DA calculation methodology

Moz calculates DA using a machine learning model that considers multiple factors, including:

• Linking root domains: The number of unique domains linking to a website.
• Total number of links: The overall quantity of inbound links.
• Spam score: An assessment of the quality of links, identifying potentially spammy backlinks.

The algorithm compares these factors against Moz's extensive index of the web, resulting in a DA score that reflects a site's ranking strength in relation to other websites.

What does the DA score tell you

DA is a comparative metric, meaning its value is best understood relative to other sites. This means it's a good signal of how likely you are to outrank your competitors.

For instance, a DA of 40 might be considered strong in a niche industry but average in a highly competitive sector. Therefore, SEO professionals should use DA to benchmark against competitors rather than as an absolute measure of success.

How to check your DA score

You can use this quick DA checker we've created for a quick reference before you continue reading the article. (You can always find the tool through this link) 

Limitations and criticisms

• Not a Google ranking factor: It's essential to recognize that DA is not used by Google in its ranking algorithm.
• Potential for manipulation: Since DA heavily weighs backlinks, there's a risk of artificially inflating scores through link schemes. Such practices can lead to penalties and should be avoided.
• Relative nature: DA is a comparative metric, i.e. a score's significance varies across industries. It's crucial to benchmark against direct competitors rather than aiming for an arbitrary high score.

Domain Rating (DR) by Ahrefs

While Moz's DA is widely recognized, other SEO tools offer similar metrics.

Ahrefs' domain rating (DR) measures the strength of a website's backlink profile on a logarithmic scale from 0 to 100, focusing only on backlinks. DR is used by most of link builders for measuring domain authority. It's the most used domain authority metric by SEO professionals.

While the exact algorithm is not publicly disclosed, Ahrefs has shared enough detail to give a reasonably clear picture of how it works.

How DR is calculated

Here’s a simplified breakdown based on what Ahrefs has disclosed:

1. Count referring domains

• Ahrefs looks at how many unique domains (not just links) are pointing to the target website.

• Only dofollow links are counted in DR.

• Multiple links from the same domain do not provide significantly more value than a single one.

2. Evaluate the authority of those domains

• The strength of each referring domain is factored in.

• If you get a link from a domain with high DR, it will influence your score more than a link from a low-DR domain.

3. Logarithmic scale

• Like Google's original PageRank, Ahrefs uses a logarithmic scale.

  • That means going from DR 20 → 30 is much easier than DR 70 → 80.

  • As a result, higher numbers require exponentially more high-quality links.

4. Passing DR through outgoing links

• DR is passed through outbound links.

• If a high-DR site links out to hundreds of domains, each one gets a smaller share of its authority.

• Therefore, being one of few outbound links from a strong site is more valuable than being one of many.

This is also the main reason most link builders use the referring domains in/out ratio as a backlink quality metric.

5. Site-wide metric

• DR is calculated at the domain level, not the page level (unlike PageRank).

• So every page on a site with DR 80 will reflect that same score, regardless of the content or inbound links to that specific page.

What DR does NOT measure

• Topical relevance of the links

• Traffic or content quality

• Spam risk (although Ahrefs does have spam detection in other tools)

• Internal linking structure of your site

How to check your DR

You can check your website DR using Ahrefs' free DR checker tool. For historical DR data, you might consider subscribing to their service.

Authority Score by Semrush

Semrush's Authority Score (AS) is a compound metric evaluating a domain's overall quality, considering factors like backlink signals, organic traffic, and spam indicators. It combines multiple data points to provide a single score on a 0–100 scale.

How AS is calculated

While the exact details of how AS is calculated are not public, here's what we know are used:

• Backlink data: Number and quality of referring domains and backlinks.
• Organic search traffic: Estimated search traffic volume.
• Spam signals: Ratio of toxic links or other spam indicators.

How it works

Semrush uses a machine learning model that considers:

• The quantity and authority of referring domains
• Link follow/nofollow status
• Referring domain relevance and trust signals
• Organic traffic trends and visibility
• Link profile health and presence of toxic links

Interpretation

Scores range from 0 to 100, where:

• 0–20: Low authority
• 21–50: Moderate authority
• 51–70: Strong authority
• 71–100: Very strong authority

Citation Flow (CF) and Trust Flow (TF) by Majestic

The main Majestic metrics used to assess domain-level authority are Citation Flow (CF) and Trust Flow (TF).

CF is score from 0–100 that predicts how influential a URL or domain might be based on the number of sites linking to it. It assesses the backlink quantity and power.

TF is a score from 0–100 that predicts how trustworthy a site is based on its backlink profile.

How CF is calculated

1. Link quantity: The more links a page has, the higher its potential Citation Flow.

2. Link influence weighting: Links from pages with higher Citation Flow contribute more to a target page’s CF.

3. Recursive flow: CF is transferred through links — if Page A links to Page B, and A has high CF, B gains CF, too.

How TF is calculated

Seed sites: Majestic starts with a hand-curated set of trusted seed sites (e.g., BBC, .gov, .edu, CNN).

Link proximity: Pages that are closer (in link distance) to these trusted sites receive higher Trust Flow.

Link inheritance: Trust "flows" through backlinks. If a trusted site links to a page, that page gains trust, which it can then pass on.

Damping: Trust decays with distance. A site that’s two or three links away from a trusted source may still benefit, but the Trust Flow is reduced.

CF/TF ratio: Health indicator

The ratio of Citation Flow to Trust Flow is often used to judge the quality of a backlink profile. Some link builders use it as their main domain authority metric.

• 1:1 to 2:1 → Natural, healthy backlink profile

• 3:1 or higher → Possibly manipulative or spammy links

• High TF, moderate CF → Often signals fewer but high-quality backlinks

Criticism

Citation Flow (CF) and Trust Flow (TF) face criticisms from link builders. CF is susceptible to manipulation through spammy or purchased links. TF aims to assess trust by measuring proximity to a proprietary, undisclosed set of trusted seed sites, but this lack of transparency and bias toward established or English-language domains can lead to under-representation of regional websites. 

However, the recent Google patent reveals did prove that Google does use a similar algorithm that uses seed sites used by TF.

While both metrics provide valuable insights, especially when used together (CF/TF ratio), they should not be used in isolation but rather alongside other tools like Moz’s Domain Authority, Ahrefs’ Domain Rating, manual link audits, and traffic analysis.

Impact of domain authority metrics on rankings

While Google does not use domain authority metrics (DA, DR, CF/CT) as a direct ranking factor, studies have shown correlations between higher scores and better search rankings. This suggests that the scores can serve as a useful proxy for assessing a site's ranking strength. However, it's worth noting that metrics such as unique referring domains seem to correlate more with rankings.

1. Ahrefs Domain Rating (DR)

Multiple large-scale studies show a modest positive correlation between DR and Google rankings.

For example, Ahrefs’ own 2022 analysis of 1,000,000 search results found a Spearman correlation of about 0.13 between Domain Rating and a page’s Google ranking (among the top 20 results). The study also suggests it's more worthwhile to focus on unique referring domains.

An earlier industry-wide study of 11.8 million Google results likewise concluded that Ahrefs DR “strongly correlates” with higher rankings on Google’s first page. According to Onely's research, that correlation is 0.14.

In practice, this means sites with higher DR tend to rank slightly better on average, while DR is far from the only ranking factor.

2. Moz Domain Authority (DA)

Recent research finds a measurable (though small) correlation between DA and actual Google rankings.

Onely’s 2022 experiment found Domain Authority had an average correlation ~0.16 with Google rank (slightly higher than DR’s correlation).

In another analysis, Moz’s own data science team (2019) calculated a Spearman correlation around 0.12 between DA and Google SERP positions (using ~16,000 random keywords). That Moz study noted DA showed the strongest correlation with rankings compared to similar third-party metrics (pointing out Ahrefs’ DR and Majestic’s Flow metrics in that dataset). These correlation values (~0.12–0.16) indicate that higher DA generally accompanies higher rankings, but the relationship is not particularly strong.

3. Semrush Authority Score (AS)

Semrush’s recent Ranking Factors 2024 study indicates a stronger correlation for this metric than seen with others.

In that study (300k SERP results analyzed), Authority Score emerged as a top correlated factor, with a reported correlation around 0.21 between a domain’s Authority Score and its Google ranking position. This made Semrush’s domain authority score the #1 domain/link-based factor .

In practical terms, Semrush observed that sites with higher Authority Scores tend to occupy higher organic positions, reflecting the cumulative effect of strong backlinks and other trust signals.

4. Majestic Citation Flow & Trust Flow (CF/TF)

Studies find that these Flow metrics also correlate with Google rankings, though somewhat less strongly than the likes of DA/DR/AS.

For instance, an analysis published in 2019 (comparing multiple tools’ metrics) found Majestic’s Trust Flow had a ~0.10 correlation and Citation Flow about ~0.09 with Google search position. These values indicate a positive but relatively weak correlation.

More research is needed to study the correlation of >CF/TF ratio to search rankings.

How link builders should use this knowledge

• Competitor analysis: Comparing DA/DR/AS/CF/TF scores can help identify competitors' strengths and weaknesses.
• Link-building prioritization: Targeting backlinks from high-DA/DR/AS/CF/TF sites can enhance a website's authority.
• Performance tracking: Monitoring DA/DR/AS/CF/TF over time can indicate the effectiveness of SEO strategy.

Comparative analysis: Which one you should use