What is the difference between CSV and XLSX?
How the two table formats differ, upload screens, and switching safely
A web dashboard says "upload your product list as CSV", and what you have is an .xlsx file built in Excel, with coloured headings and a total row at the bottom. Or a program has given you a .csv file and you want to turn it into a tidy table before sending it to a colleague. Both open in Excel, both look like tables. So where is the difference?
The difference lies in how much of the file is kept. XLSX stores the whole workbook: sheets, formulas, colours, column widths. CSV saves only the values in the cells, as plain text with a separator between them. In other words, moving from XLSX to CSV deliberately leaves things behind. This article covers what is left behind, when that is a problem and when it is exactly what is wanted.
What survives a save, and what does not
The diagram below shows a three-month stationery list: one sheet per month, a bold coloured heading row, and a formula at the bottom that adds up the total. Save the same file as CSV and what remains is the plain text on the right.
Two things are easy to miss. First, only the sheet that is open at that moment goes into the CSV. Excel tells you so in a dialog as you save, but people click through it quickly and often notice days later that the other months are missing. Second, the total cell no longer holds a formula, only the result at the time of saving: 790. Change one of the amounts afterwards and the total will not update itself.
What each part of your workbook becomes in CSV
If your file contains any of the following, decide whether that part matters to you before switching to CSV. The third column makes the decision easier by asking who will read the file.
| In the workbook | After saving as CSV | Is it a problem? |
|---|---|---|
| Several sheets | Only the open sheet remains | Yes; save the other sheets one by one |
| Formula (such as =SUM) | The result at save time becomes a fixed value | Yes, if the data will change |
| Bold text, colour, borders | Gone entirely | Yes for a human reader, no for a program |
| Merged cells | The value stays in the first cell, the others are emptied | Merged headings can scramble the columns |
| Column widths, frozen rows, filters | Gone | No; the opening program sets its own layout |
| Charts, images, comments | Gone | Yes if you are preparing a report |
| Cell values | Kept in full | No; this is the actual data |
Why does a system insist on CSV?
Online shop dashboards, bulk SMS and email services, contact import screens and the bulk upload sections of accounting software mostly ask for CSV. The reason is simple: these systems do not look at a table, they read data line by line. Colours, formulas or a second sheet mean nothing to them, while the plainness of CSV means the file is read the same way whichever program wrote it.
When you meet a screen like that, the safest route is to download the sample file the system offers (often a link such as "download template") and put your data into it. Do not rename the column headings, do not reorder them and do not leave blank rows in between: the system usually recognises columns by name or by position. If the upload is rejected, check the row number in the error message; the problem is usually a cell on that row.
The right order for saving as CSV in Excel
First switch to the sheet holding the data you want to upload, because only that sheet will go into the CSV. Then go to File > Save As and pick "CSV (Comma delimited)" from the file type list. If the list also shows an option starting with "CSV UTF-8" and your data contains letters such as é, ñ or ü, choose that one instead.
When you press Save, Excel may tell you two things: that only the current sheet will be saved, and that some features may be lost. Both are expected warnings. The part to watch comes afterwards: from that moment the open file is a CSV, and any formatting you add will be lost when you close it. So always produce the CSV as a copy of your XLSX file and keep your real work saved as .xlsx.
If Excel is not installed, or you want to split several sheets in one go, the XLSX to CSV tool does the same job in your browser: by default it converts the first sheet, and with "All sheets" selected it downloads each sheet as a separate CSV named after it. In Google Sheets, the File > Download menu has a .csv option.
The other way: when to turn a CSV into XLSX
A CSV that came out of one program and is going into another should stay as it is; converting in between only adds a step. But if a person will read it, if calculations will be done on it, or if it is going to someone by email, converting to XLSX makes life easier: the recipient double-clicks and the columns open in the right place, number cells are ready to add up, and formulas and colours can be added and kept.
The CSV to XLSX tool works out the separator from the file and leaves codes that begin with zero as text. If you see broken letters or lost leading zeros when you open a CSV in Excel, the cause and the fix are covered in a separate guide.
Why accented characters break in CSV files, and how to fix them →
When there are too many rows: Excel's ceiling
A CSV file has no row limit of its own; an export from a database can run to millions of lines. A single Excel worksheet, however, holds at most 1,048,576 rows and 16,384 columns. Open a bigger CSV in Excel and the file does not load completely: rows beyond the limit never appear, and if you save the file in that state, those rows are gone.
Converting such a file to XLSX is no way out either, because the same ceiling applies to an XLSX sheet. Very large data should either stay as CSV or be split into parts first. The Split CSV tool cuts the file into pieces of the row count you choose and can repeat the heading row in every piece, so each part opens in Excel on its own.
Microsoft: Excel specifications and limits →
Frequently asked questions
Do I lose formulas if I save XLSX as CSV?
The formula itself is lost; its result stays. CSV writes the value shown in the cell at the moment of saving, and that value will not update if you later change the cells it came from. If you need the formulas to keep working, keep your main file as XLSX.
I saved as CSV and the colours and other sheets are gone. Can I get them back?
Not from the CSV file; that information was never written into it. If your original XLSX file still exists, carry on from there. That is why a CSV should always be made as a copy rather than saved over the original.
A system wants CSV. Which option do I choose in Excel?
Under File > Save As, choose "CSV (Comma delimited)" as the file type, or the option starting with "CSV UTF-8" if your list has it. If the upload screen asks for a particular separator or encoding, compare your file with the sample in its help pages.
Can I put several sheets into one CSV file?
No, a CSV holds a single table. If the sheets share the same columns, you can first stack them on one sheet in Excel and then save; otherwise each sheet has to be its own CSV. The XLSX to CSV tool gives you every sheet as a separate file in one go.
Which is smaller, CSV or XLSX?
It depends. XLSX is a compressed package inside, so for large tables it can carry the same data in less space than CSV; for very small tables the package's own overhead can make it larger. Choose by where the file is going, not by its size.
Last updated: October 12, 2026