Keyword Difficulty Explained: Why Ahrefs, Semrush, and Moz Give Different Scores
Keyword difficulty scores from Ahrefs, Semrush, Moz, and KWFinder are not interchangeable. This guide explains how each tool calculates KD, why the numbers differ, and how to use them correctly.
The problem with KD scores
If you run the same keyword through Ahrefs, Semrush, and Moz, you will get three different numbers. All three claim to measure “how hard is it to rank for this keyword.” None of them are wrong — they are measuring different things and calling them the same thing.
This guide explains exactly what each tool measures, why the scores diverge, and how to use them without being misled.
Ahrefs KD: the backlink model
Ahrefs Keyword Difficulty is the most literal of the three. It measures the average number of referring domains (unique websites linking to a page) pointing to the top-10 SERP results.
The score is a 0–100 range with this approximate translation:
| Ahrefs KD | What it means | Required referring domains (approx.) |
|---|---|---|
| 0–10 | Minimal competition | 0–10 referring domains to top-10 |
| 10–30 | Low competition | 10–35 referring domains |
| 30–50 | Moderate | 35–80 referring domains |
| 50–70 | Hard | 80–200 referring domains |
| 70–90 | Very hard | 200–500 referring domains |
| 90–100 | Near-impossible for new sites | 500+ referring domains |
Advantage: this is the most literal and actionable metric. You can set a specific link-building target: “I need links from X domains to compete.”
Limitation: it ignores content quality, brand authority, and topical relevance. A keyword dominated by thin content from high-DA sites may have a high Ahrefs KD despite being theoretically rankable with better content.
Semrush KD: the authority model
Semrush Keyword Difficulty uses Semrush’s own Authority Score (a composite metric of referring domains, referring IPs, and follow/nofollow ratio) for the top-10 results. It correlates with Ahrefs KD directionally but often scores differently in practice.
Semrush KD tends to be higher than Ahrefs KD for the same keyword when:
- The top results come from very high-authority sites with diverse link profiles
- The keyword has strong brand signals (navigational intent mixed in)
And lower when:
- The top results have many links but from fewer diverse domains (link farms)
Moz KD: the Page Authority model
Moz Keyword Difficulty uses the average Page Authority (PA) of the top-10 SERP results. PA is itself a composite score based on Moz’s link index.
The key difference from Ahrefs: Moz measures the authority of the page, not the count of referring domains to that page. A page with 10 backlinks from very authoritative sites might have a higher PA than a page with 100 backlinks from low-authority sites.
Moz KD tends to be lower than Ahrefs KD for keywords where the top results have many low-quality backlinks, and higher for keywords where the top results have few but very high-authority backlinks.
KWFinder KD: the weighted PA/DA model
KWFinder (Mangools) uses a weighted average of Page Authority and Domain Authority for the top-10 results, with additional adjustments for the number of referring domains and social signals. It generally correlates most closely with Moz KD.
The SERP inspection shortcut
For any keyword you’re seriously considering, skip the KD score and look at the actual SERP:
- Do any of the top-10 results come from sites with no clear topical authority in your niche? (Forum posts, off-topic brand mentions, thin affiliate pages) — these are displaceable.
- Is there a featured snippet? If yes, who owns it — can you create a better, more structured answer?
- Are any of the top-10 results older than 3 years with few backlinks? Old content decays in authority.
These manual checks take 5 minutes per keyword and are more predictive than any KD score.
Using KD correctly
- Use KD as a filter, not a final decision
- Compare KD within a single tool — never compare Ahrefs KD 35 to Moz KD 35 and assume they are equivalent
- Pair any KD score with manual SERP inspection for your top targets
- For new sites: target KD 0–20 in Ahrefs, KD 0–30 in Semrush. These are attainable within 6–12 months.
Related reading
- Keyword difficulty glossary entry — the concise definition
- Search volume — the demand side of the equation
- SERP — understanding what you are competing against
- Ahrefs review | Semrush review | Moz Pro review — how each tool’s KD feature works in practice