Will my leading zeros and long numbers survive?

Nothing is retyped. Every value is carried through as text: 00412 keeps its zeros, 1.10 keeps its trailing zero, and a sixteen-digit number never becomes 1.23E+15.

Why does opening a CSV in Excel directly go wrong?

Excel retypes values as it opens a CSV: 00123 loses its zeros and SEPT2 becomes a date. This converter writes each value into the workbook as text, so Excel displays what the file said.

How big a file can it convert?

Up to Excel’s own limit of 1,048,576 rows, and up to 200 MB of input — the workbook is built in this page, so the ceiling is your browser’s memory and it is stated, not discovered.

Can I convert more than one sheet into the workbook?

Not here — one CSV becomes one sheet. To stack several files first, use the combine tool and convert the result.

Is my file uploaded anywhere?

No. The conversion runs inside this page: when you convert CSV to Excel here, the browser is told to block outbound connections, so nothing can be sent to another server. Keep the Network tab of your developer tools open while you convert CSV to Excel: every request in it goes to this site, and none carries your file.

Convert CSV to Excel — without your numbers changing

Turn a CSV into an .xlsx workbook in your browser. Leading zeros, long numbers and dates stay exactly as written, and the file never leaves your device.

CSV to ExcelRunning on your device

Source

Your CSV

or drag one here · it stays on your device

Result

Add a CSV file. The preview shows the table exactly as the workbook will hold it.

How this works

Add your CSV. Drop the file onto the page, or pick it.
Check the preview. The grid is the table the workbook will hold.
Download. Save the .xlsx and open it in Excel.

What it cannot do

Every cell is written as text, deliberately: asking Excel to guess types is how leading zeros die. Format columns as numbers inside Excel if you need arithmetic.
Excel itself stops at 1,048,576 rows; a longer file converts up to that limit and says so rather than truncating silently.
One sheet per file. Combining several CSVs into one workbook is a different job (combine, then convert).

Questions

Will my leading zeros and long numbers survive?: Nothing is retyped. Every value is carried through as text: 00412 keeps its zeros, 1.10 keeps its trailing zero, and a sixteen-digit number never becomes 1.23E+15.
Why does opening a CSV in Excel directly go wrong?: Excel retypes values as it opens a CSV: 00123 loses its zeros and SEPT2 becomes a date. This converter writes each value into the workbook as text, so Excel displays what the file said.
How big a file can it convert?: Up to Excel’s own limit of 1,048,576 rows, and up to 200 MB of input — the workbook is built in this page, so the ceiling is your browser’s memory and it is stated, not discovered.
Can I convert more than one sheet into the workbook?: Not here — one CSV becomes one sheet. To stack several files first, use the combine tool and convert the result.
Is my file uploaded anywhere?: No. The conversion runs inside this page: when you convert CSV to Excel here, the browser is told to block outbound connections, so nothing can be sent to another server. Keep the Network tab of your developer tools open while you convert CSV to Excel: every request in it goes to this site, and none carries your file.

Your file is processed in this page and never uploaded — the privacy page shows how to confirm that yourself.

Common problems this solves

Excel stops at 1,048,576 rows — here is how to open the rest — Excel silently drops every row past 1,048,576, and saving makes it permanent. Open, check and query the whole file in your browser — nothing uploaded.
My CSV opened as one column — Excel used semicolons — A CSV showing everything in column A was saved with semicolons by a European Excel. Why, how to tell, and how to convert it without retyping a number.
Excel removed the leading zeros — get them back and keep them — Excel turns 00412 into 412 on open; saving makes it permanent. Check whether your file still has its zeros, keep them, or put them back — in your browser.
Excel shows #### instead of the value — what it means and what is really in the cell — Number signs across a cell mean the column is too narrow, or the value became a negative date. The value is intact; here is how to see it and keep it.
Excel says the file format and extension don't match — what the file really is — The warning means the file is not what its name says — usually a web page of a table, or plain CSV, named .xls. How to tell, and how to get the rows out.
The Shopify orders CSV: one row per item, blank order fields, and how to make it one row per order — Shopify writes one row per line item and fills the order columns only on the first. Fill them down and keep phones and totals as written, in your browser.
Export Stripe data to CSV: which export to take, what it holds, and matching payouts to the bank — Which Stripe export to take, what the balance and payout files hold, how to match payouts to the bank, and loading one into a database or BI tool.
The Google Analytics CSV: comment lines above the heading, a second table underneath, and numbers with commas — A Google Analytics download starts with # lines, ends with a second table, and writes numbers with commas and dates as 20240131. Make it one clean table.
The Xero report export: organisation and date-range lines above the headings, and totals in the rows — A Xero report export carries the organisation name and date range above the headings and total rows in the data. Strip them here for a plain table.
A Spanish CSV: semicolons, decimal commas, and identifiers that lose their zeros — Spanish exports use semicolons and a decimal comma. The quieter problem is reference numbers with leading zeros that a spreadsheet throws away on opening.
A Brazilian CSV: semicolons, decimal commas, and amounts with R$ attached — Brazilian exports use semicolons and a decimal comma, and often carry the currency inside the amount column, which stops any of it converting. What to do.