Excel drops leading zeros and turns long numbers into 1.23E+15 when opening a CSV
Last reviewed 2026-09-19
Excel guesses a type for every column when it opens a CSV. A value like 00123 looks like the number 123, so the zeros are dropped, and an ID longer than 15 digits is rounded because Excel stores numbers with about 15 digits of precision. Once the file has been saved from Excel, the lost zeros cannot be recovered from that copy.
Messages you might see
- “Leading zeros disappear from ZIP codes, phone numbers and product codes”
- “Long numbers show as 1.23457E+15”
Why it happens
- Double-clicking a CSV makes Excel apply its own guess to each column.
- Codes, IDs and phone numbers are stored as numbers instead of text.
- The file was already saved from Excel, which wrote the changed values back.
Try this first (free, no tool needed)
- Go back to the original CSV, not the copy Excel saved.
- Use Data > From Text/CSV, click Transform Data (or Edit) and set the affected columns to Text before loading.
- Or format the target columns as Text first, then paste or import into them.
- For a single value you type by hand, put an apostrophe in front (’00123).
If that didn't work
These tools run in your browser. Your file is read and repaired on your own device and is never uploaded.
CSV-to-Excel Rebuilder
Builds a valid .xlsx from a CSV or TSV where numbers, ISO dates and TRUE/FALSE become real Excel values, while leading zeros, long IDs and phone numbers stay text. It has optional bold frozen headers, filter buttons and auto-sized columns. It needs the original CSV, as the zeros are already gone from a file re-saved by Excel.
Open CSV-to-Excel RebuilderCommon questions
Can it restore zeros that Excel already removed?
No. If the zeros were lost when the file was saved, they are no longer in it. Go back to the original CSV.
Is my file uploaded anywhere?
No. The tool runs as a program inside your own browser, so the file is read and repaired on your device and never sent to a server.