The short answer
For most new APIs and web applications, JSON is the practical default — it is lighter, maps directly onto JavaScript objects, and nearly every modern language has fast native support for it. XML earns its keep in older enterprise systems, documents that need mixed content (text interleaved with markup), or environments that require strict schema validation and namespaces.
Syntax and size
JSON represents data as key-value pairs, arrays, and primitive types with minimal punctuation. XML wraps every value in opening and closing tags plus optional attributes, which is more verbose — an XML document carrying the same data as JSON is typically 30–60% larger before compression. That difference matters more on mobile networks and high-volume APIs than it does for a one-off config file.
Parsing and tooling
JSON parses directly into native data structures in JavaScript, Python, and most modern languages, with no separate schema step required. XML parsing is more work but comes with mature tooling for validation (XSD, DTD), transformation (XSLT), and namespaces — useful when documents pass between many independent systems that need to agree on structure ahead of time.
When XML is still the right call
- Legacy enterprise integrations (SOAP, many banking and government systems)
- Documents with mixed content, like a paragraph of text with inline formatting tags
- Strict contractual schemas validated before ever reaching application code
- Systems that already standardized on XML and have no reason to migrate
Working with both
You do not have to choose blind — format the JSON you are inspecting first with the JSON Formatter, and if you need to hand it to an XML-only system, convert it directly with JSON to XML (and back with XML to JSON) rather than hand-writing the conversion.
Quick FAQ
Is JSON always faster than XML?
Parsing JSON is usually faster because it maps directly to native data structures, but the real-world difference is negligible for small payloads. It becomes more noticeable at scale, in high-volume APIs and large documents.
Can JSON do everything XML can?
No. JSON has no native support for comments, mixed content, namespaces, or attribute-based metadata the way XML does. For document-style content, XML is often still the better fit.
Should I convert an existing XML API to JSON?
Only if you control both ends and have a reason to, such as smaller payloads or easier client parsing. If the format is dictated by a partner or legacy system, converting on your side with a tool is usually simpler than rewriting the API.