JSON formatter and validator

Written byOleh Soloninupdated 25 September 2026

JSON validation

JSON is valid

{
  "project": "UniTools",
  "version": 1,
  "active": true
}

Readable output, or a straight answer about what is wrong

Paste raw JSON to indent it readably, or find out exactly where it breaks. Runs in your browser, so payloads never leave the machine.

API responses arrive as one unbroken line. Formatting adds the indentation that makes structure visible, which is usually all you need to spot the field that is missing.

When JSON is invalid, the message tells you where parsing stopped. The culprit is almost always a trailing comma or a single quote — both legal in JavaScript, neither legal in JSON.

How to use

Paste the JSON — parsing happens in the browser and nothing is sent anywhere.
Switch between indented and minified: indentation is for reading and diffing, minified is for the wire.
If the JSON is invalid, start from the position in the message — that is where the parser stumbled. The cause itself can sit earlier: an unclosed quote or bracket only shows up where the text stops making sense.

What indentation costs

Indentation changes the writing, not the data. The same three-field object is 48 characters minified and 61 indented — and the difference is entirely spaces and line breaks.

So the wire gets the minified form and a file in the repository gets the indented one: only a file split into lines diffs line by line, and a single changed value then shows as one line rather than the whole document.

Both figures come from the same JSON.stringify call the tool itself makes, on the sample in the field. On this sample the overhead is 27 per cent; on other documents it depends on nesting, key names and values, and can be much smaller or many times larger.
Form	Characters	Lines	What it is for
Minified	48	1	transport and storage
Indented by two spaces	61	5	reading and diffing

Five inputs JSON refuses

JSON is stricter than the JavaScript it grew out of. Four of the five rows below would be legal in a .js file and are not in a .json one — which is where almost every error message comes from.

The fifth is the leading zero. JSON forbids it inside a number because several languages read it as octal. A part number with leading zeros belongs in a string, where it survives any amount of processing.

The positions come from the same JSON.parse the tool uses. The wording of the message is written by the browser's own parser and differs between Chrome, Firefox and Safari; the position points at the same place in all three.
Input	Why it fails	Position in the message
{"a":1,}	trailing comma — legal in JavaScript, not in JSON	7
{'a':1}	single quotes — JSON requires double	1
{a:1}	unquoted key	1
{"a":1} // comment	JSON has no comments	8
{"a":01}	leading zero in a number	6

What this calculator shows

Indented, readable JSON with consistent nesting
A validation error pointing at the position that failed
Minified output when you need to shrink a payload back down

What to keep in mind

Everything runs locally in the browser; nothing is uploaded.
Strict JSON rules apply — comments and trailing commas are rejected, as any parser would reject them.

FAQs

Why is my JSON invalid when it looks fine?

Usually a trailing comma, single quotes, or an unquoted key. All three are fine in JavaScript and none are valid JSON. The other common culprit is an invisible character picked up when copying out of a document.

What does the position in the error mean?

That depends on the browser: the message may give a character offset, a line and column, or only a description. Use the position your browser reports — exact positions are not portable between parsers — and indenting the document first makes a line number more useful.

Is it safe to paste production data here?

Parsing happens in the browser; nothing is transmitted or stored. The usual care with other people's data still applies: what you would not put on a clipboard does not belong in a form field either.

Can JSON contain comments?

No — the standard has none. Where comments genuinely matter, people reach for YAML or JSON5. The common workaround is a field named something like _comment: formally it is ordinary data, and parsers simply carry it along.

Are large numbers lost?

Above roughly nine quadrillion, yes — that is where JavaScript's precision ends. A nineteen-digit identifier comes back with its last digits altered and no warning at all, which is why such keys belong in strings.

What is the point of minifying?

Not shipping whitespace. On the sample here indentation adds 27 per cent; on a large API response it adds considerably more. For a human reader that cost is worth paying, for machine-to-machine traffic it is not.

Worked example

A response worth reading

Input: {"user":{"id":42,"roles":["admin","editor"]}}

Output: Nested object across nine indented lines

Note: One line becomes a shape you can scan. With fifty fields instead of three, that difference stops being cosmetic.

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