Excel Tools

Open a CSV in Excel without losing leading zeros or long numbers

The tool scans your CSV for the values Excel changes when it opens the file: postcodes, IDs, card numbers, gene names, codes like 1-2 or 1E5. It shows you each risky column and gives you an .xlsx where those values are stored as text.

No file at hand?

How to open a CSV without losing data

  1. Choose the CSV. The encoding and delimiter are detected automatically.
  2. Review the columns at risk. For each one you see how many values Excel would change and real examples from your file, with what Excel would show instead.
  3. Pick what stays text. Risky columns are ticked for you. Tick others, or force every column to text.
  4. Download the .xlsx and open it. Protected values are stored as text cells, so Excel cannot convert them. The other columns keep their numbers so you can still sum them.

Private by design: no upload

Lab results, customer IDs and account numbers are exactly the data you should not upload to a random website. Here the file is read and written in your browser, in a background thread; nothing leaves your device. You can check it in the Network tab of your browser's developer tools.

What Excel changes when it opens a CSV

A CSV has no column types. When you double-click one, Excel guesses the type of every value and converts it on the spot, before you can see the original. These are the conversions this tool looks for:

The gene name problem

The best-documented damage comes from genetics. Excel silently turns gene symbols such as MARCH1 or SEPT2 into dates, and those files end up in published papers. An analysis of more than 11,000 articles from 2014 to 2020 found errors of this kind in 31% of the supplementary files that contained gene lists in Excel (Revista Pesquisa FAPESP, Retraction Watch). The HUGO Gene Nomenclature Committee ended up renaming about 27 genes, for example MARCH1 to MARCHF1 and SEPT1 to SEPTIN1, so that spreadsheets would stop mangling them. Older datasets still use the old names, and the conversion also depends on the language of Excel.

Doesn't Excel have a setting for this now?

In 2023 Microsoft added Automatic data conversion options (File › Options › Data) that let you switch off removing leading zeros, keeping only 15 digits, converting to scientific notation and converting to dates. They help, with limits: Microsoft lists them for Microsoft 365 and Excel 2024 (on Mac from version 16.77), they do not apply to Power Query imports or while macros run, and they only change the computer where you set them (Microsoft Support). A colleague with an older Excel, LibreOffice or a different default still sees the damage. An .xlsx with text cells looks the same for everyone.

How to do it by hand in Excel

This tool does those steps for you and shows which columns needed them. For a general conversion with numbers and dates, use CSV to Excel, which protects the same columns by default. If accents look like “é”, fix the CSV encoding first.

Known limitations

Frequently asked questions

Is this tool really free?

Yes. It is 100% free, with no usage limits, no account and no sign-up. There is no paid plan and no feature is locked.

Is my CSV uploaded to a server?

No. The file is read and analysed inside your browser. The page only downloads its own code and the open-source SheetJS library from this site; the contents and names of your files are never sent anywhere. If you accept analytics cookies, Google Analytics receives anonymous usage data such as page views, never your data or file names.

Why does Excel remove leading zeros from my CSV?

When Excel opens a CSV it guesses the type of every value. 00123 looks like a number, so it is stored as 123 and the zeros are gone. Changing the cell format afterwards does not bring them back, because the zeros were never stored. The fix is to make Excel treat the column as text before the data is loaded, which is what the .xlsx from this tool does.

Why do my 16-digit numbers end in zeros?

Excel keeps only 15 significant digits in a number. A 16-digit card or account number such as 4111111111111111 becomes 4111111111111110, and anything longer loses more digits. Numbers with 12 to 15 digits keep their value but are displayed in scientific notation (8.41235E+12), and that is what gets written if you save the file as CSV again.

Why does Excel turn gene names like MARCH1 or SEPT2 into dates?

Excel reads MARCH1 as 1 March and SEPT2 as 2 September, and converts them silently. A study of genetics papers found errors from this in 31% of supplementary files with gene lists, and the HGNC renamed several genes (MARCH1 became MARCHF1, SEPT1 became SEPTIN1) to avoid it. Codes such as 1-2 or 3/4 are converted the same way.

Doesn't Excel have a setting for this now?

Recent Microsoft 365 and Excel 2024 builds for Windows and Mac have Automatic data conversion options to switch off removing leading zeros, truncating long numbers, scientific notation and date conversion. They are not available in older versions, they do not apply to Power Query imports or while macros run, and they only help on the computer where they are changed. An .xlsx with text cells opens correctly everywhere.

What does “stored as text” mean in the file?

Each protected value is written as a text cell with the text number format (@), so Excel, LibreOffice and Google Sheets show it exactly as it was in the CSV. Excel may mark these cells with a small green triangle (“number stored as text”); that is expected and can be ignored.

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