Short answer: CSV is plain text with commas separating values — nothing else. XLSX is a full spreadsheet file that also stores formatting, formulas, multiple sheets, and data types. CSV is a data format; XLSX is an application format.
What CSV actually is
A CSV (comma-separated values) file is just text. Open one in a plain text editor and you'll see exactly what's stored — rows of values separated by commas, one row per line. There's no bold text, no column widths, no multiple tabs, and no way to store a formula. Every application interprets the raw text itself.
What XLSX actually is
An XLSX file is technically a small zip archive containing several XML files describing sheets, styles, formulas, and metadata. That's why it can hold things a CSV never can:
- Multiple sheets in one file
- Cell formatting — bold, colors, borders, number formats
- Formulas that recalculate
- Explicit data types, so a ZIP code stays text and a date stays a date
When to use CSV
CSV is the right choice when you need a simple, universal data interchange format — exporting from a database, feeding data into a script, or sharing rows between systems that don't care about formatting. Almost every tool on earth can read a CSV.
When to use XLSX
XLSX is the right choice when a human is going to open the file directly and work with it — applying formulas, formatting a report, or keeping multiple related tables in one document. It's also the safer choice whenever a column contains leading zeros, long ID numbers, or dates, since XLSX preserves the exact type instead of guessing.
Converting between the two
Going from XLSX to CSV loses formatting, formulas, and multiple sheets — only the raw values survive. Going from CSV to XLSX is safer, since you're adding structure rather than removing it. csvtoexcel.io handles that conversion for free, directly in your browser.