Convert a CSV to Excel (XLSX)

How to convert CSV to Excel

1. 1. Open the converter in your free workspace. Your CSV is read into the browser’s memory — never sent over the network.
2. 2. Drop your .csv file. Sigmera detects the delimiter and encoding automatically, so accents and special characters survive.
3. 3. Download the .xlsx workbook. The result opens in Excel, Google Sheets, Numbers, or LibreOffice.

Why convert in the browser?

Most online converters upload your file to a server you can’t see. Because Sigmera converts the file locally with the browser’s built-in engine, there is no server to upload to and nothing is stored — which satisfies GDPR data-minimization because zero bytes leave your device.

Need the opposite direction? Use the same workspace to convert XLSX to CSV when an importer refuses .xlsx and wants plain comma-separated text.

If the file is going to a person rather than a system — a report to print, sign off, or attach to an email — skip the workbook and turn the CSV straight into a PDF table instead.

CSV to XLSX: what changes in the file

A CSV is plain text — one line per row, values split on a delimiter, no cell types and no second sheet. An .xlsx workbook is a zipped XML package that records each cell’s type and number format. That difference is why double-clicking a CSV into Excel mangles data: Microsoft’s own import guidance notes that Excel applies its default data format settings when it opens a text file, which is what drops leading zeros from ZIP codes and rewrites long phone numbers in scientific notation. Running the CSV through a converter writes real cell values instead, so the columns arrive intact.

The only hard ceiling is Excel’s worksheet limit of 1,048,576 rows by 16,384 columns. Under that, a browser CSV to Excel converter is bounded by your own device memory rather than an upload size cap — there is no server in the path.

Most people convert because something downstream needs a workbook. If the goal is tidy data rather than the format itself, it is worth removing duplicate rows and standardizing phone numbers before you convert, so the .xlsx you hand over is already clean. See all the cleaning tools for the rest.

Why a CSV opens in one column in Excel

Double-clicking a .csv sometimes drops every field into column A. That is a delimiter mismatch, not a corrupt file. Microsoft documents that Excel uses the Windows list separator as the delimiter for .csv files, so a comma-delimited export opens unsplit on a machine whose regional list separator is a semicolon — the default across much of Europe.

There are two ways past it. Inside Excel, import the file rather than opening it: Get & Transform Data > From Text/CSV lets you pick the delimiter before the data lands. Or convert the file to .xlsx first — the converter reads the delimiter out of the CSV itself, so the workbook arrives already split into columns whatever your regional settings say. The walkthrough of Excel’s own import path covers the first route, and what a .csv file actually stores explains why the separator is a convention the format never declares.

Why Excel turns CSV values into dates

The other thing that breaks on a double-click is anything shaped like a date. Product codes such as 12/2, ratios such as 1-2, and measurements such as 3/4 get read as calendar values, and Microsoft states the behaviour plainly: “12/2 changes to 2-Dec”, and there is no setting to turn it off. The fix Microsoft documents is to make the cells text before the value lands — either by preformatting them as Text, or by choosing the column’s data format during import instead of opening the file directly.

Converting the CSV to .xlsx applies the same idea earlier in the chain. The converter writes each field into the workbook as the value it already was, so the column is decided before Excel gets a chance to guess. If the file is on its way somewhere else anyway — a CRM, a mail merge, an accounting import — it is worth doing the cleanup at the same time: tidy the email column and swap stray values across a column while the file is open, then export once. The write-up of what CSV and Excel each actually store covers which of the two formats a given importer really wants.

Why a CSV shows garbled characters in Excel

The third failure has nothing to do with columns or dates. Names and addresses come through as é, ’, or a row of question marks, because Excel decoded the bytes with the wrong character set. A .csv carries no declaration of its own encoding, so something has to guess — and Microsoft is direct about what the guess depends on: a UTF-8 file opens normally only if it was saved with a BOM (byte order mark), and otherwise has to be imported rather than opened.

The Power Query documentation states the rule from the other direction. Its File Origin dropdown exists precisely because the character set is not inferred, and UTF-8 is only inferred if the file starts with a UTF-8 BOM. That is the whole bug in one sentence: an exporter that writes clean UTF-8 without a BOM — which most databases and CRMs do — hands Excel a file it will read as Windows-1252, and every accented character breaks.

Converting to .xlsx settles the question before Excel sees the file. Sigmera reads the BOM when there is one; when there is none it validates the bytes as strict UTF-8 and only falls back to Windows-1252 or UTF-16 if that validation fails — a decision made from the file’s actual contents, not from your machine’s regional settings. The text is decoded once, then written into the workbook as Unicode, where the encoding is no longer a guess anyone has to make. If you are re-exporting afterwards, the reverse converter writes UTF-8 back out, and the anatomy of a .csv covers why the format never records its own encoding in the first place.