Open a CSV in Excel Without Losing Leading Zeros (Zip Codes, IDs, Phone Numbers)

Excel drops leading zeros because it converts anything that looks like a number when you double-click a CSV. To keep them, import the file with the columns set to Text, change Excel's automatic data conversion setting, or convert the CSV to an .xlsx with those columns stored as text. Only the last one works for whoever opens the file next, because the type is saved in the file.

Source: Microsoft Support, Keeping leading zeros and large numbers and Excel specifications and limits, read 3 November 2026.

What Excel changes when it opens a CSV

A CSV stores every value as text; RFC 4180 defines rows and fields, not types. Excel guesses the types, and the guesses that damage contact lists are:

In the CSV	After a double-click
02108 (zip code)	2108
+447911123456	447911123456
4111111111111111 (16-digit ID)	4111111111111110, shown as 4.11E+15
1-2 (a unit or size)	2 January
1E5 (a part code)	100000
=SUM or -text	Read as a formula

The second problem is the worse one: if someone saves the file as CSV again, the changed values are written back. A list that went through one colleague's Excel can reach your CRM with every New England zip code four digits long.

Fix 1: import instead of double-clicking

In Excel, use Data > From Text/CSV, choose the file, click Transform Data, and set the zip, phone and ID columns to Text before loading. This works, but it has to be repeated every time and by everyone who opens the file.

Fix 2: change Excel's setting

Current Microsoft 365 builds have File > Options > Data > Automatic Data Conversion, where you can untick removing leading zeros and converting long numbers. It changes your Excel only, so the colleague you send the file to still sees the damage.

Fix 3: send an .xlsx with text columns

An .xlsx records each cell's type, and a cell stored as text is never converted. The CSV to Excel tool does this in your browser:

1. Upload or paste the CSV (comma, semicolon and tab files are read, up to 25 MB).
2. Press Check and convert. It lists each column Excel would change, with an example and a count: leading zeros, numbers over 15 digits, date-like values, scientific notation, leading plus signs, formula starts, TRUE/FALSE and thousands separators.
3. Keep the default (only those columns as text, so quantity and revenue stay numbers) or store every column as text.
4. Download the .xlsx and send that instead of the CSV.

Phone numbers are a special case worth a second look: once they're safe, format phone numbers in Excel covers getting them into one consistent format.

If the zeros are already gone

They can't be recovered from the file itself. For US zip codes, find and replace in CSV with regular expressions on, ^(\d{4})$ replaced by 0$1, puts the zero back on four-digit values in the zip column only. For account numbers or IDs, re-export from the source system.

Limits

The tool writes one sheet per file, Excel's 1,048,576-row limit applies, and cells over 32,767 characters are cut. Dates stay as text rather than becoming Excel dates. It checks the conversions listed above, not every regional setting (a semicolon-and-comma-decimal locale can still surprise you). And it keeps values intact; it doesn't tell you whether the email addresses in them work.

Where SimpleVerifier fits

If the sheet is a contact list, upload the CSV or the .xlsx at /verify: every address gets a live mailbox check, and valid, risky and invalid download as separate files with your other columns kept. Verify emails in Excel walks through the round trip. The first 100 checks are free with a Google sign-in. Pro is $39 a month with the first 100,000 checks a month at full speed, then slower, never cut off, and one-time packs of 10,000 checks for $15 never expire. See pricing.