Convert CSV to Excel without uploading the file

The file you are converting is the problem

Nobody converts a CSV of sample data. They convert the export from the CRM, the finance system, or the analytics tool, and those files carry names, email addresses, salaries and revenue figures. In most organisations, uploading one of those to an unvetted third party is a policy breach whether or not anything bad happens.

The conversion itself is not the hard part. Writing an xlsx workbook is a well-defined format operation that runs perfectly well in a browser tab.

Getting the parsing right

Most CSV bugs come from the edge cases rather than the format: quoted fields containing commas, embedded newlines inside a quoted value, a UTF-8 byte order mark on the first header, and semicolon delimiters from European locales. The tools here handle all four, and the validator will point at the row when something is genuinely malformed.

The other common surprise is Excel reformatting values on open, turning long account numbers into scientific notation and text that looks like a date into a date. Writing a real xlsx with explicit cell types avoids that, which is the main advantage over simply renaming the file.

Will long numbers still turn into scientific notation?

That happens when Excel guesses the type of a value in a plain CSV. Writing a real xlsx workbook lets the cell type be set explicitly, which is why converting properly is better than renaming the extension.

Does it handle semicolon-delimited CSV?

Yes. The delimiter is selectable, which covers the semicolon-separated exports common in European locales where the comma is the decimal separator.

Can it do multiple sheets?

The CSV to Excel conversion produces a single sheet, since a CSV is a single table by definition. Going the other way, the Excel tools read every sheet in a workbook and let you pick.