Free YAML Validator & Formatter Online

Validate YAML with exact line-number errors, normalize indentation, and convert between YAML and JSON - all in your browser with syntax highlighting.

100% free
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Runs in your browser

Input (YAML, or JSON for "Convert from JSON")

0 lines 0 chars

Output ( - )

Guide

What is YAML?

YAML (YAML Ain't Markup Language) is a human-friendly data format used heavily for configuration: Docker Compose, Kubernetes, CI pipelines, and many app config files. It favours indentation and minimal punctuation over the brackets and quotes of JSON, which makes it readable but also sensitive to small mistakes. This tool validates YAML, reports the exact error line, and converts between YAML and JSON.

YAML uses spaces, never tabs

The single most important YAML rule: indentation must use spaces, not tabs. The spec forbids tabs for indentation, and a stray tab is one of the most common reasons a file fails to parse. Consistent two-space indentation is the usual convention. The validator normalises indentation and flags structural problems so you can fix them quickly.

Common YAML errors

Using tabs instead of spaces for indentation.
Inconsistent indentation levels within the same block.
Missing the space after a colon in a key-value pair.
Unquoted strings that contain special characters like ":" or "#".
Misaligned list items under a key.

YAML vs. JSON

YAML and JSON can represent the same data, and in fact every JSON document is valid YAML. JSON is stricter and more compact, ideal for machine-to-machine transfer, while YAML is easier for humans to write and supports comments, which JSON does not. Converting between them (as this tool does) is handy when a system expects one format but you have the other.

Last updated: July 2026

Frequently Asked Questions

What is YAML?

YAML (YAML Ain't Markup Language) is a human-readable data serialization format commonly used for configuration files, CI/CD pipelines (GitHub Actions, Docker Compose), and data exchange.

What are common YAML syntax errors?

The most common YAML errors are incorrect indentation (YAML uses spaces, never tabs), missing colons after keys, improper quoting of special characters, and duplicate keys.

Can I convert YAML to JSON?

Yes. Use the Convert to JSON button above. YAML is a superset of JSON, so most valid YAML converts cleanly to JSON.

What is the difference between YAML and JSON?

Both are data formats. YAML is more human-readable with cleaner syntax for config files. JSON is more universally supported and required by many APIs. YAML supports comments; JSON does not.

Does YAML use tabs or spaces?

YAML only allows spaces for indentation, never tabs. Using tabs is the most common cause of YAML parse errors. Two spaces per indent level is the widely accepted standard.