Let's give Grammarly its due first. Their new AI Humanizer is a real product, not a gimmick. It offers 4 preset styles, supports 6 languages (English, Spanish, French, German, Portuguese, Italian), and lets you create custom voice profiles using a 200-word writing sample. The output quality is genuinely good. Text comes out sounding natural, fluid, and polished. If all you care about is making AI text *read* better, Grammarly's humanizer delivers.

But reading better and passing detectors are two completely different things. And that's where Grammarly falls apart.

Modern detectors like Turnitin, GPTZero, and Originality.ai analyze three core signals: perplexity (how surprising word choices are), burstiness (variation in sentence length and structure), and token predictability (how expected the next word is). Grammarly's humanizer produces clean, well-structured output, which is exactly the kind of statistically uniform text that detectors flag. Making text sound *better* doesn't mean making it sound *human* to an algorithm. Those are different objectives, and Grammarly optimizes for the wrong one.

The Perkins et al. (2024) study, published in the International Journal of Educational Technology in Higher Education, found that AI detectors start with a baseline accuracy of just 39.5%. But here's what matters: paraphrasing and surface-level humanization (Grammarly's approach) only reduce detection by about 17.4% on average. That's nowhere near enough. Tools that restructure deeper statistical patterns push bypass rates dramatically higher because they target the actual signals detectors measure, not just the tone and readability.

Our benchmark results confirmed this. Grammarly's humanizer achieved roughly an 8% pass rate on GPTZero's strict thresholds. Against Turnitin? Flagged consistently. Originality.ai? Flagged. Copyleaks? Flagged. The output *sounded* great. It just didn't *pass*. And that's the core problem: Grammarly built a humanizer optimized for quality, not for the statistical fingerprints that detectors actually scan.

At $12/month (annual billing) or $30/month (monthly) for Grammarly Pro, you're paying for a tool that's excellent at making AI text read well but fundamentally misaligned with detection bypass. The Weber-Wulff et al. (2023) study tested 14 AI detection tools and found all of them scored below 80% accuracy, meaning there's a real window to exploit. Grammarly's humanizer doesn't exploit it. It produces beautiful text that still gets caught.

Grammarly is excellent at what it does. What it does just isn't bypassing AI detection. And for the 140+ million users who now have access to its AI features, that distinction matters more than ever.

We tested five dedicated detection-bypass humanizers against the same essay that Grammarly's humanizer failed to protect. Each tool was evaluated against Turnitin, GPTZero, Originality.ai, Copyleaks, and ZeroGPT.

Here's what's interesting: Grammarly's output quality is legitimately competitive with some tools on this list. Its humanizer produces clean, natural-sounding text. But output quality and detection bypass are two different things. Where Grammarly's approach is "make the text sound more human to a reader" (essentially a paraphraser, not a humanizer), these tools think in terms of detection patterns: restructuring at the token level, introducing natural variance in sentence rhythm, and mimicking the statistical fingerprint of human writing. The Perkins et al. (2024) study confirmed this distinction matters. Tools targeting perplexity and burstiness simultaneously push bypass rates far beyond what quality-focused humanization can achieve.

UndetectedGPT is the clear winner. A 96% bypass rate with 9.2/10 readability at $19.99/month (with a free tier to test first). It costs more than Grammarly Pro's $12/month annual price, but Grammarly can't bypass detectors at all. It uses genuine pattern restructuring: analyzing perplexity, burstiness, and token prediction sequences, then rewriting them to mirror natural human writing. Under 5% on Turnitin. Under 4% on Originality.ai. Consistent results across dozens of tests. Multiple humanization modes let you dial intensity based on your use case.

StealthGPT takes a different approach, generating new undetectable content rather than rewriting existing text. At $32/month, it achieves roughly 82% bypass rate with decent customization. The AppSumo community rates it 3.64 out of 5 (22 reviews), and Trustpilot gives it 4.0 out of 5 (181 reviews). The main drawback: output quality. Multiple reviewers describe the text as "gibberish" or "nonsensical" even when detection scores are low. You might bypass the detector, but your professor (or editor) will notice something's off.

Undetectable AI includes a built-in AI detector alongside its humanizer, so you can verify results before submitting. At $19/month for 10,000 words (with a free tier of 250 words), it delivers an 80% bypass rate. The dual detection-plus-humanization feature eliminates the need for separate detector subscriptions. The Scribbr independent test found even the best detectors only achieve 76% accuracy, so an 80% bypass rate clears most hurdles. A solid middle-ground option, though it trails UndetectedGPT by 16 points.

WriteHuman is tuned for editorial and blog content. It starts at $18/month. Its 78% bypass rate handles most content platform detectors, and the output has a clean, professional tone. If your use case is exclusively content marketing (not academic), WriteHuman is a specialized pick.

HIX Bypass rounds out the list at $11.99/month with a 75% bypass rate and multilingual support. If you write in multiple languages and need humanization across all of them, HIX Bypass fills a niche that most competitors ignore. Its 7.5/10 readability is the lowest on the list, but adequate for non-English content where the bar is different.

A few things jump out immediately. First: UndetectedGPT actually bypasses detectors. At $19.99/month, it costs more than Grammarly Pro's $12/month annual, but Grammarly's humanizer can't bypass detectors at all. You're paying for a tool that solves the actual problem. Second: every single tool on this list obliterates Grammarly's ~8% pass rate. Even HIX Bypass at the bottom of the table delivers a 75% bypass rate. That's nearly 10x Grammarly's performance.

The Perkins et al. (2024) study quantified why this gap exists. Basic paraphrasing (Grammarly's approach) reduced detector accuracy by 17.4% on average. But dedicated humanization tools that target perplexity and burstiness simultaneously pushed bypass rates to 96%. The difference isn't incremental. It's the difference between a tool that occasionally confuses a detector and a tool that systematically defeats one.

For context: the Liang et al. (2023) Stanford study found that AI detectors flag 61.22% of TOEFL essays by non-native English speakers as AI-generated. If you're an ESL writer already fighting uphill against biased detectors, Grammarly's 8% pass rate is functionally zero. You need a tool that consistently clears the bar, not one that polishes your grammar while leaving you exposed.

Here's the thing: UndetectedGPT costs $19.99/month for the Starter plan (with a free tier to test first). Yes, that's more than Grammarly Pro at $12/month (annual). But Grammarly's humanizer can't bypass detectors at all. UndetectedGPT makes AI text pass detectors with a 96% bypass rate. Only one of those matters when your professor runs your essay through Turnitin.

Bypass rate: 96% vs Grammarly's ~8%. That's not a comparison. It's a different sport. Against Turnitin, UndetectedGPT consistently scored under 5% AI detection. Against Originality.ai (which scored 76% on the Scribbr independent test, making it the toughest publicly benchmarked detector), under 4%. Against GPTZero, clean passes on strict mode. Grammarly's humanizer failed all three, despite producing output that sounded great.

Since [Turnitin](/blog/turnitin-ai-detection-guide) launched dedicated AI bypasser and humanizer detection on August 27, 2025, the bar has risen even further. Turnitin specifically trained their system to catch text processed through humanizer tools. A tool that was already failing at detection bypass (like Grammarly's humanizer) has zero chance now. UndetectedGPT's pattern-level restructuring works at a deeper level than what Turnitin's new system targets.

Readability: 9.2/10. This is where the comparison gets nuanced. Grammarly's humanizer also produces high-quality output. But UndetectedGPT matches that quality *while also passing detectors*. You don't have to choose between text that sounds good and text that passes. UndetectedGPT delivers both.

The tool also offers multiple humanization modes tailored to different use cases: academic, blog, general. You're not stuck with a one-size-fits-all button. Need Turnitin-proof academic writing? There's a mode for that. Light cleanup for a LinkedIn post? Dial it down.

You're getting the highest bypass rate on the market (96%) with comparable output quality. Grammarly Pro is cheaper but can't bypass a single detector. That's the whole point.

Grammarly users are typically looking for a simple, reliable tool. Here's the direct advice for each situation.

If you want the highest bypass rate available. UndetectedGPT at $19.99/month costs more than Grammarly Pro, but Grammarly can't bypass detectors at all. UndetectedGPT delivers a 96% bypass rate. The Perkins et al. (2024) study confirmed that pattern-level humanization dramatically outperforms surface-level editing. There's a free tier to test before you pay.

If you want advanced customization. StealthGPT at $32/month offers granular control over humanization strength. Its 82% bypass rate is solid, though at $32/month you're paying $12 more than UndetectedGPT while giving up 14 points on bypass rate. Whether that trade-off makes sense depends on how tough your detectors are.

If you process large volumes. Undetectable AI at $19/month handles bulk content well with an 80% bypass rate and a built-in detector for verification. Good for agencies and content teams.

If your focus is marketing content. WriteHuman at $18/month specializes in editorial and blog content. Its 78% bypass rate handles most content platform detectors.

If you need multilingual support. HIX Bypass at $11.99/month supports multiple languages at 75% bypass rate. Valuable if English isn't your only output language.

If you're an ESL student. Pay attention. The Liang et al. (2023) Stanford study found that AI detectors flag 61.22% of TOEFL essays by non-native English speakers as AI-generated. Nearly 1 in 5 were unanimously misclassified by all 7 detectors tested. You're already fighting uphill against biased detectors. You need UndetectedGPT's consistent sub-5% Turnitin scores, not Grammarly's 8% pass rate.

Can you use both Grammarly and a humanizer? Absolutely. Use Grammarly for grammar and clarity, then run the output through UndetectedGPT for detection bypass. They solve different problems. Just don't expect Grammarly to solve both.

The easiest test: try UndetectedGPT's free tier. Process the same text through both tools and compare against any detector. The difference speaks for itself.