How to open a CSV in Excel correctly

Steps

1. Easiest: convert the file with the CSV to Excel converter, then open the .xlsx. Leading zeros and text values stay as they are.
2. Or in Excel, open a blank workbook and choose Data, then From Text/CSV.
3. Pick the file and check that the file origin says 65001: UTF-8 (this fixes accents) and the delimiter matches your file.
4. Press Transform Data if some columns need to stay as text, such as postcodes and phone numbers, and set their type to Text.
5. Press Load.

What goes wrong

Leading zeros disappear: 01234 becomes 1234, which breaks postcodes, phone numbers and product IDs.

Long numbers turn into scientific notation. Excel keeps only 15 digits of a number, so a 16-digit card or ID number is damaged for good once it is saved as a number.

Some text is read as dates. A value like 1-2 can become 2 January, and some gene names have famously been mangled this way.

Accents and non-English letters show as strange characters when the file is UTF-8 but Excel assumes an older encoding.

Semicolons and commas

In countries that write decimals with a comma, Excel expects a semicolon between columns, so a comma CSV opens as one long column. The converter on this site looks at the first line and chooses the right separator, and also understands quoted values that contain commas.

Going the other way

To save an Excel sheet as CSV for a database or an upload form, use the Excel to CSV converter. It writes a UTF-8 marker by default so Excel shows accents correctly when you open the CSV again.