Best AI detector for teachers

What a teacher can buy directly, and what it costs

Every figure below is from the vendor's own pricing page on 17 August 2026. Compare on volume rather than headline price: a class of thirty submitting 1,500-word essays is 45,000 words per assignment, which is the number that decides whether a plan is workable.

• GPTZero — free tier covers 10,000 words a month with a 10,000-character limit per scan, three advanced scans and a batch upload limit of three files. Premium raises that to 300,000 words a month, a 50,000-character scan limit and 50 files per batch; Professional to 500,000 words, 150,000 characters and 250 files. Its own site prices in local currency, so check the figure shown to you. LMS integration with Canvas and Google Classroom is a Professional-tier feature.
• Copyleaks — Personal is $13.99/month billed annually at $167.88, giving 1,200 unified credits, which the page states as up to 300,000 words. Pro is $74.99/month billed annually at $899.88 for 12,000 credits, up to 3,000,000 words, and includes 25 user seats — that tier is a department buying together, not one teacher. AI and plagiarism run in a single report.
• Originality.ai — Pro is $14.95/month, or $12.95/month billed annually, for 2,000 credits a month. One credit is 100 words for AI detection alone and two credits per 100 words if you scan for plagiarism at the same time, so 2,000 credits is 200,000 words of AI-only scanning. Credits expire at the end of the month.
• Winston AI — Essential is $18/month, or $10/month billed annually, for 100,000 credits a month; Advanced $29 or $16 for 200,000; Elite $49 or $26 for 500,000. All paid plans take up to 200,000 characters per scan. The free tier is a 14-day trial with 2,000 credits.
• Grammarly — its AI detector is free, and claims "99% detection accuracy" and a first-place ranking on the RAID benchmark, while also stating on the same page that "No AI detector is 100% accurate" and that it should not be a standalone verification method.
• QuillBot — the AI detector is included with Premium at $4.17/month billed annually, alongside the paraphraser, plagiarism checker and, notably, an AI humanizer. Free access to the detector is limited.

Why Turnitin is not in that list

Because you cannot buy it as an individual teacher. Turnitin's AI writing indicator requires an institutional Turnitin Originality licence, and an administrator can switch the indicator off, in which case nothing is processed for AI writing at all.

This is worth knowing before you spend anything: if your institution already licenses Turnitin, the detector is in the Similarity Report you already have access to, and buying a second tool duplicates it. If it does not, no personal purchase gets you Turnitin. The distinction also explains why teachers get contradictory answers about their own LMS — Canvas, Blackboard and Google Classroom ship no AI detection of their own, so what appears in an assignment is whichever tool the institution connected.

The number that should decide your choice

Words per month against your actual marking load. Detector pricing is metered, and the tools differ by an order of magnitude at similar prices.

Work it out before you subscribe: classes multiplied by students multiplied by average assignment length multiplied by assignments per month. Three classes of thirty, 1,500 words, twice a month is 270,000 words — above GPTZero Premium's 300,000 only in a heavy month, comfortably beyond Originality.ai Pro's 200,000 of AI-only scanning, and inside Winston AI's Advanced tier. Also check the per-scan ceiling separately from the monthly one, because a long dissertation chapter can exceed a scan limit while sitting well inside the monthly allowance.

What the accuracy claims can and cannot tell you

Every vendor publishes a number, and none of them publishes a test you can reproduce. Treat the figures as marketing until a vendor names its corpus.

Turnitin is the exception that shows what a checkable claim looks like: it states a false positive rate under 1% for documents containing over 20% AI writing, and says it validates that before each model release against more than 700,000 academic papers written before ChatGPT was released. Grammarly's 99% cites an external benchmark, RAID, and its own page still says no detector is 100% accurate. Others state a percentage with no corpus attached. Under 1% is also not zero: across 300 marked essays a year, a sub-1% false positive rate still puts a handful of honest students in front of you with a flag on their work.

How to use a detector without generating an injustice

Treat the score as a prompt to look more closely, never as a finding. Turnitin's own position is that the percentage "should not be used as the sole basis for action or a definitive grading measure by instructors", and that is the vendor of the strictest tool speaking.

• Announce which tool you use and what a flag means, before the first assignment rather than after the first accusation.
• Ask for process evidence rather than a confession: version history, drafts, notes and sources. A document built over days looks different from one that arrived complete.
• Run a second detector before acting. Disagreement between two tools on the same text is common and is itself information.
• Watch for the known false-positive patterns — non-native English, heavily edited prose, formulaic academic structure, and short submissions where there is little text to judge.
• Keep the score out of the grade. It is evidence in a conversation, not a deduction.

The cheaper alternative to buying a detector

Assignment design. Detection is a check applied after the fact, and it is the only part of this problem where you have no control over the error rate.

Work that references class discussion, requires a specific text you chose, asks for a personal process account, or is built in stages with drafts submitted along the way is both harder to generate wholesale and easier to verify honestly. That is not a substitute for a detector in every setting, and it costs preparation time rather than money. But it moves the question from "can I prove this was AI" — which no current tool answers with certainty — to "can this student account for their own work", which is answerable in a five-minute conversation.

Disclosure

We make an AI writing tool, so read this page with that in mind. Appvior's AI Humanizer scores text for AI-likeness and rewrites it; it is built for writers editing their own drafts, not for grading, and it is not one of the detectors compared above.

It is not an institutional tool, it keeps no class records, and it makes no claim to be evidence of anything. We have listed the detectors a teacher would actually consider, including several that compete with the detection half of our own app, and quoted prices from their pages rather than our impressions of them.