7 Free Format Conversion Tools Every Developer Needs in 2026
Every developer knows the pain. You're knee-deep in a project, and suddenly you need to convert a CSV into JSON. Then someone sends you GPX data from their GPS watch. Then your data scientist drops an Excel file full of nested objects and asks you to make it "web-ready."
You could write a throwaway script for each one. Or you could use the right tool for the job and move on with your life.
I've been curating ElysiaTools.com — a free, browser-based toolkit for developers — and today I want to walk you through the 7 format conversion tools I reach for most often. No signup. No rate limits. No "please upgrade."
1. CSV to JSON Converter
Converting CSV to JSON sounds simple until you have 47 columns, some values with commas inside them, and a header row in a different language.
This tool handles the edge cases: custom delimiters (comma, tab, semicolon, pipe), quote character handling, whitespace trimming, and the ability to skip empty lines. You get a clean JSON array out, plus metadata showing input lines, output rows, headers detected, and the delimiter that was used.
The output even tells you what it processed and when — useful if you're running this as part of an automated pipeline.
Best for: Ingesting spreadsheet exports, processing API responses, prepping data for JSON-native frontends.
2. CSV to Markdown
The inverse problem: you have CSV and you need Markdown table format. Maybe for a README. Maybe for a GitHub issue. Maybe for a Notion doc.
This one converts CSV to Markdown table with optional table borders, header row support, and a delimiter auto-detector. Paste your data, pick your options, and copy the table directly.
Best for: Documentation, GitHub READMEs, any Markdown-based writing workflow.
3. Base64 Converter
Base64 shows up everywhere: API authentication headers, embedded images in CSS, data URIs, JWT tokens. This tool handles encoding and decoding with three options that matter in real work:
-
URL-safe mode — replaces
+and/with-and_, which is required for Base64 in URL parameters and JWTs. - Line breaks toggle — adds RFC 2045 line breaks (76 chars per line) for email compatibility, or keeps it clean for data URLs.
- UTF-8 vs ASCII — respects multi-byte characters when encoding, which matters for non-English text.
Best for: Working with APIs that use Basic Auth, embedding small assets inline, decoding JWT payload sections.
4. HTML to Markdown Converter
Scraping content? Migrating a site? Extracting article text from a paywalled HTML mess? This is the tool.
It uses the Turndown library under the hood — the same library many Node.js projects use — and gives you granular control: convert links, images, tables, and lists independently, or preserve whitespace exactly as it appears in the source.
The output is clean Markdown you can paste straight into a static site generator, a documentation tool, or a content management system.
Best for: Content migration, web scraping post-processing, converting HTML email bodies to readable text.
5. GPX to GeoJSON Converter
This one is more specialized, but if you work with GPS data, fitness apps, or mapping tools, you'll appreciate it.
GPX (GPS Exchange Format) is the standard output of Garmin, Strava, and most GPS devices. GeoJSON is what Leaflet, Mapbox, and most web mapping libraries expect. This converter bridges the gap — handling waypoints (points of interest), tracks (recorded paths with elevation data), and routes (planned paths).
The output is a proper GeoJSON FeatureCollection with typed features. You get the geometry (Point or LineString) and a properties object with name, elevation, segment info, and type.
Best for: Fitness app data visualization, mapping dashboards, GIS pipelines.
6. XLSX JSON Transformer
This one is deceptively powerful. It's not just "Excel to JSON" — it's a bidirectional transformer with smart nested object handling.
JSON → XLSX mode: Upload a JSON array, flatten nested objects using dot-notation paths (e.g., user.address.city), and get a spreadsheet where each column is a flattened path. Arrays can be stored as indexed columns (tags.0, tags.1) or as JSON strings — your choice.
XLSX → JSON mode: Upload an Excel file, read the flattened columns, and restore the original nested structure. Value types are auto-detected and restored — strings that look like numbers become numbers, true/false become booleans, "null" becomes null.
Best for: ETL pipelines, passing spreadsheet data to APIs, importing configuration data into apps.
7. JSON Schema to Zod Schema Converter
This is the tool I wish I'd had six months ago.
JSON Schema is the standard for describing JSON API responses and configuration files. Zod is the TypeScript-native runtime validation library everyone's using in 2026. Until now, you had to manually translate between them — or use a buggy automated script.
This converter handles the full JSON Schema spec: objects, arrays, enums, oneOf/anyOf/allOf composition, $ref with circular reference detection, string formats (email, URL, UUID), numeric constraints (min/max), and even nullable fields. You can output just the Zod schema, or schema plus an inferred TypeScript type via z.infer<typeof schema>.
You can name the root schema, pick camelCase or PascalCase naming, and include or exclude description comments.
Best for: Validating API responses, building type-safe configuration systems, adding runtime validation to TypeScript projects.
Bonus: OpenAPI to TypeScript Generator
Since we're on the topic of schema-driven development — this one takes an OpenAPI or Swagger specification (JSON or YAML) and generates TypeScript interfaces, types, and operation contracts.
It extracts component schemas as exported interfaces, generates per-endpoint parameter groups (path, query, header, cookie), builds request body types, and creates discriminated union response types that include the HTTP status code and the response data shape.
You can choose flat exports or a namespace wrapper, pick PascalCase or camelCase, use interface or type declarations, and include JSDoc descriptions from the spec.
Best for: Bootstrapping API client libraries, generating type-safe fetch wrappers, keeping your frontend types in sync with your backend specification.
The Stack
| Tool | What it does | Try it at |
|---|---|---|
| CSV to JSON | CSV → JSON with full config | elysiatools.com/en/tools/csv-to-json |
| CSV to Markdown | CSV → Markdown table | elysiatools.com/en/tools/csv-to-markdown |
| Base64 Converter | Encode/decode Base64 | elysiatools.com/en/tools/base64-converter |
| HTML to Markdown | HTML → clean Markdown | elysiatools.com/en/tools/html-to-markdown |
| GPX to GeoJSON | GPS data → GeoJSON | elysiatools.com/en/tools/gpx-to-geojson |
| XLSX JSON Transformer | Excel ↔ JSON (bidirectional) | elysiatools.com/en/tools/xlsx-json-transformer |
| JSON Schema → Zod | JSON Schema → Zod code | elysiatools.com/en/tools/json-schema-to-zod-schema-converter |
| OpenAPI → TypeScript | OpenAPI spec → TS types | elysiatools.com/en/tools/openapi-to-typescript-generator |
Why These Tools Are Different
Most online converters are ad-supported, capped, or stuffed into a "freemium" model where you hit a paywall after three uses. The tools on ElysiaTools are:
- 100% free — no account required
- Client-side by default — your data doesn't leave your browser (except for file output)
- No rate limits — run them as many times as you want
- Open source — the codebase is on GitHub
The conversion tools especially benefit from this: they're deterministic, stateless operations — exactly the kind of thing that runs perfectly in the browser without a server.
If you found this useful, ElysiaTools has 1,500+ tools across categories like data processing, document generation, audio processing, and data visualization. Bookmark it, share it with your team, and feel free to contribute on GitHub.
What conversion pain points do you hit most often? Drop them in the comments — I might just build a tool for it.
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