How most free checkers actually work
Under the hood, a free checker is usually one of two things. The first is a generic readability or grammar engine — the same class of tool that flags passive voice and long sentences in any document. It counts errors, measures sentence length and word variety, then maps those numbers onto a band scale that was never designed for them.
The second is a large language model with a thin prompt — something close to "rate this IELTS essay from 1 to 9". The model has read enough of the internet to produce a confident-sounding number, but a one-line instruction gives it nothing to anchor to. It guesses, and it guesses generously.
Why that misses IELTS entirely
An IELTS band is not a verdict on "good writing". It is the average of four specific, equally-weighted descriptors: Task Response, Coherence & Cohesion, Lexical Resource, and Grammatical Range & Accuracy. Each has detailed band boundaries, and the difference between a 6 and a 7 on any one of them is a precise, documented judgement — not a vibe.
A grammar score knows nothing about whether you answered the question. A readability metric can't tell a clear position from a fence-sitting one, or a developed paragraph from a list. So the tool measures what it can measure — surface mechanics — and quietly ignores the half of the rubric that actually decides your band.
The inflation incentive
There's also a softer reason free checkers run high: a tool that flatters gets shared. Tell someone their essay is a Band 8 and they feel good, screenshot it, and recommend you. Tell them it's a 6 because they ignored half the prompt, and they close the tab. Over thousands of users, the generous tool wins on traffic — accuracy isn't what's being optimised.
None of this requires bad intent. A vanilla model genuinely leans optimistic, and nobody building a free widget is penalised for being kind. But the result is the same: a number that feels reassuring and means very little.
A grammar checker is not an examiner
This is the core problem. Two of the four criteria — Task Response and Coherence & Cohesion — are about meaning and structure, not correctness. They ask whether your argument actually addresses the task, whether each paragraph carries one clear idea, and whether the reader can follow your logic without effort.
You cannot reach those judgements by counting commas. A flawless paragraph that answers the wrong question is still a Task Response problem. A grammar engine has no way to see that, which means half the rubric is invisible to it from the start.
The inconsistency tell
Here's a quick way to catch a weak checker: paste the same essay twice. A generic LLM with no fixed rubric will often hand back different bands on different runs — a 6.5 now, a 7.5 in ten minutes — because nothing constrains it to mark the same work the same way.
An examiner doesn't do that. The rubric exists precisely so the same essay scores the same band regardless of who's marking or when. If a tool can't reproduce its own result, it isn't measuring anything stable — it's improvising.
What accuracy actually requires
Honest marking isn't magic; it's discipline. It means a prompt grounded in the official descriptors, not a single sentence. It means scoring the four criteria separately, so a strong vocabulary can't paper over a missed task. It means quoting your own words back as evidence for each judgement, so you can check the reasoning rather than trust a bare number. And it means returning the same band for the same essay every time.
That last point matters most. Consistency is the difference between a measurement and a guess. If a tool can't reproduce its own marks, nothing it tells you is safe to act on.
A two-minute test you can run on any checker
You don't have to take anyone's word for this. Run two experiments on whatever tool you're using. First, deliberately worsen your essay — break a few sentences, add some obvious errors — and re-check it. If the band doesn't drop, the tool isn't really reading your grammar either.
Second, delete your answer to half the question. Cut the paragraph that handles the second part of the prompt, leave the rest polished, and re-check. An honest checker drops your Task Response sharply. A flatterer won't notice the missing half at all — and that single test tells you everything about whether the number is worth keeping.