Convert a large CSV to Excel (.xlsx)

Pick Excel .xlsx under Convert format and you get a real workbook rather than a renamed CSV. Excel export holds the whole sheet in memory while it builds, so keep it under about a million rows — which is also Excel's own ceiling. Over that, split the file first and convert each part.

Why not just rename the file to .xlsx?

Because it is not one. A CSV is plain text; an .xlsx is a zipped XML package. Renaming produces a file Excel refuses to open, or opens with a warning about corrupted content. Conversion has to actually rewrite the data.

The formatting traps

Opening a CSV directly in Excel silently mangles several common column types, and this is the main reason to convert deliberately rather than double-click:

• Leading zeros vanish. Postal codes, account numbers and country codes like 01234 become 1234.
• Long numbers become scientific notation. A 16-digit card or order reference turns into 1.23457E+15, and the original digits are gone.
• Dates get reinterpreted. 03/04/2026 is read as March or April depending on your machine's locale, and gene names like SEPT1 famously become dates.

These are Excel's import heuristics, not defects in your file. If a column must survive exactly as written, the safest route is to convert here and then format that column as Text in Excel before any editing.

Other formats

If the destination is a program rather than a person, JSON or NDJSON is usually a better target than a spreadsheet — no 1,048,576-row ceiling and no type guessing.

Common questions

Why is my .xlsx conversion failing on a big file? The workbook is assembled in memory. Above roughly a million rows it will not complete — split the file and convert each part.

Will leading zeros survive the conversion? Values are written as they appear in the CSV. Excel may still display them differently; format the column as Text before editing to be certain.

Can I convert several CSVs into one workbook? Not currently. Each conversion produces one workbook with a single sheet.