Most "CSV vs JSON" comparisons are vibes. Here are real numbers, measured on a real dataset.
Setup: 200,000 rows x 12 columns, Node v22.18.0, no streaming tricks.
| Metric | Result |
|---|---|
| CSV file size | 20.5 MB |
| JSON file size | 49.3 MB |
| Size ratio | CSV is 2.4x smaller |
| CSV to JSON speed | ~75,234 rows/sec |
| JSON to CSV speed | ~70,908 rows/sec |
| Round-trip | Lossless |
Why JSON is bigger: repeated key names on every object, plus escaping of quotes and non-ASCII characters. For log files and export pipelines the size gap translates directly into bandwidth and storage costs.
What to use: JSON for APIs and nested structures; CSV for storage, spreadsheets and analytics imports. If your data is a flat table, CSV wins on size and tooling.
I built a free in-browser converter that does JSON↔CSV↔TSV plus a bunch of text utilities, 100% client-side (your data never leaves the machine): https://instant-data-converter.vercel.app
Full guide with methodology: https://instant-data-converter.vercel.app/guides/csv-vs-json
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