Keyword gap analysis is the one job in SEO where the free tools genuinely cannot substitute for the paid ones. Google Search Console knows everything about the queries your site already appears for and nothing whatsoever about your competitors, and the gap is defined entirely by what they rank for and you do not.
That means you are buying access to someone's keyword index, and the size and freshness of that index is the product. This list compares the thirteen on that basis rather than on interface, because the interface is not what you are short of.
How we checked this list
All thirteen were loaded on their live sites in August 2026 and screenshotted, so every tool here resolved and is real. Anything that 404ed or turned out to be a parked domain was cut before writing.
Index sizes are quoted only where the vendor publishes them, and they should be treated as marketing rather than measurement. What we can say reliably is which tools cover which markets and where the free tiers stop, so each entry says that instead. Once a gap list turns into published pages, the follow-up job is watching whether they move, which is covered in how to track keyword rankings.
Keyword gap is not content gap
These two get used interchangeably and are different jobs with different tools.
| Keyword gap | Content gap | |
|---|---|---|
| The question | Which queries do they rank for that we do not? | Which topics have they covered that we have not? |
| The unit | A single query | A page or a subject |
| Output | A keyword list, often thousands long | A short list of articles to write |
| Best done with | A large keyword index | Judgement, plus a keyword index |
Keyword gap gives you volume, content gap gives you a plan. Most teams need the second and buy the first. Our guide to content gap analysis tools covers that half of the job.
The big-index tools
1. Semrush Keyword Gap, the feature everyone else copied
Compare up to five domains at once and get every query sorted into shared, missing, weak and untapped. The four-way split is the reason this is the standard: "missing" and "weak" are different problems, and a tool that lumps them together gives you a list you cannot prioritise.
Free tier: limited lookups per day without a subscription.
The catch: it is one module of an expensive suite, and comparing five domains on the entry plan burns through your daily limits fast.
2. Ahrefs Content Gap, cleanest data of the three majors
Despite the name, Ahrefs' Content Gap report works at the keyword level and does exactly this job. Its index is the one most practitioners trust for accuracy, and the intersection filters are more flexible than the competition's.
Free tier: none worth planning around. Webmaster Tools covers your own site only.
The catch: priced for professionals, with no monthly plan that makes casual use sensible.
3. Similarweb, traffic share alongside the keywords
Adds traffic share, SERP features, difficulty and intent to every keyword in the comparison, which turns a flat gap list into something you can sort by commercial value. If you want to see how a title will render against those SERP features before writing it, our free SERP preview tool shows it.
The catch: enterprise pricing and an enterprise sales process. Its strength is competitive intelligence generally, so you are buying more than a gap tool.
4. SE Ranking, the sensible mid-market pick
A competent gap comparison inside a suite that costs meaningfully less than the two leaders, with white label reporting if you are handing the analysis to a client.
The catch: a smaller index than Semrush or Ahrefs, which shows most in long-tail and non-English markets.
5. Serpstat, strong value for multi-domain work
Its domain-versus-domain comparison handles several competitors at once and is priced well below the majors, with particularly decent coverage of Eastern European markets.
The catch: data depth varies noticeably by country. Spot-check a handful of keywords you already know before trusting a whole export.
6. SpyFu, built around competitor history
Its whole product is competitor analysis, so the gap view is central rather than an extra tab, and the historical data showing what a competitor ranked for years ago is genuinely unusual.
The catch: US-centric. Coverage outside the US is thin enough to mislead.
7. Moz, keyword gap with a usable difficulty score
Moz's comparison is straightforward and its difficulty metric remains one of the better calibrated ones, which matters when a gap list of 4,000 keywords needs cutting to twenty.
The catch: a smaller index than the leaders, and the tool has been iterating slowly.
The cheaper and free options
8. Mangools KWFinder, the friendliest gap tool
The gap feature sits inside the least intimidating suite in SEO, and there is a free version that will let you run a real comparison rather than just look at a teaser.
Free tier: yes, genuinely usable for a single comparison.
The catch: daily lookup limits on lower plans, and thinner long-tail data than the majors.
9. RankingGap, a tool that does only this
Built for one job, comparing your site against competitors and surfacing what you lack, with none of the suite around it. If gap analysis is a recurring task and you do not want another subscription to a full platform, this is the specialist.
The catch: single-purpose means you will still need a keyword research tool alongside it, and it relies on third-party data.
10. The HOTH keyword gap tool, free with three competitors
A free tool that takes your domain plus three competitors and compares keyword types between them. It is a lead magnet, and it is still the most generous free gap comparison available.
Free tier: the whole thing, in exchange for your email.
The catch: you cannot filter or export the way a paid tool allows, and the underlying data is licensed rather than their own.
11. Rank Tracker by SEO PowerSuite, pay once
Desktop software with a free version that includes competitor keyword research, and a licence model rather than a subscription. For anyone running gap analysis occasionally, the economics are completely different from the SaaS tools above.
Free tier: yes, with saving and export restricted.
The catch: it is a desktop application that runs the queries from your machine, so large comparisons are slow.
12. Ubersuggest, cheap and adequate
Competitor keyword comparison at a low subscription price or a one-time licence, aimed at small businesses rather than agencies.
The catch: the data is noticeably less reliable than the majors. Fine for direction, poor for a client deliverable.
13. SEO Review Tools, the free spreadsheet method
Not really a product so much as a documented method for doing gap analysis with free exports and a spreadsheet. Worth knowing because it works, costs nothing, and teaches you what the paid tools are doing behind the button.
The catch: it is manual, and it does not scale past a couple of competitors.
How to run the analysis without drowning in it
Export the gap, then throw most of it away. A raw gap list against three competitors will run to thousands of keywords and the majority will be irrelevant, branded, or so competitive that appearing on the list means nothing.
Filter to keywords where at least two competitors rank and you do not, because one competitor ranking is often an accident and two is a pattern. Then filter again to positions 1 to 20, since a competitor sitting at 60 is not proving the keyword is winnable. What survives is usually twenty to fifty keywords, and that is a content plan rather than a spreadsheet.
The part that comes after the gap
A gap list is a list of pages that do not exist yet. None of the tools above will write them, and the reason most gap analyses produce nothing is that the export gets saved to a folder and the articles never get made.
That is the part Distribb handles: turning a keyword list into published articles, into your CMS, with internal links maintained as the library grows. It does not do competitor gap comparison, which is why it is not one of the thirteen above, so pair it with a tool from this list rather than instead of one. There is a 3-day free trial.
For the surrounding work, see our comparisons of keyword research tools and keyword research automation.