Does Excel warn me before dropping the rows?

Once, at open: a small notice that the file was not loaded completely. After that nothing on screen reminds you, which is how the truncated version ends up saved over the original.

If I already saved from Excel, are the rows gone?

From that file, yes — Excel wrote only what it had loaded. Go back to the original export or re-export from the system that produced it; there is nothing in the truncated file to recover from.

Can Google Sheets open it instead?

Sheets has its own ceiling of ten million cells per spreadsheet, so a wide file hits it well before a million rows. It also uploads the file, which may not be acceptable for the data involved.

How do I find out how many rows the file really has?

Run the health check on it. It reads every row and reports the count, along with columns that look inconsistent, without opening the file in a grid.

Will opening it here change anything in the file?

No. The editor keeps every value as the exact text in the file, and the download reproduces the file with only the edits you made — the same quoting and line endings, checked by tests rather than promised.

Is my file uploaded anywhere?

No. The editor reads your file straight from your own disk as you scroll, so a file past Excel’s 1,048,576 rows opens in place rather than being sent somewhere to be split up. The browser is told to refuse any outbound connection while it runs.

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Excel stops at 1,048,576 rows — here is how to open the rest

Excel can show at most 1,048,576 rows per sheet. Open a bigger CSV and it loads the first million-and-change, shows one warning, and quietly leaves the rest out — if you then save, the file on disk loses those rows for good. The file itself is fine until that save.

What actually happens to your file

The limit is a limit of the grid, not of your file. Excel reads the first 1,048,576 rows, shows a one-time notice that the file was not loaded completely, and then behaves as if that were the whole thing. Row counts, totals and filters all describe the truncated view.

The danger is the save. Save As CSV from that truncated view and Excel writes exactly what it holds, so the rows it never loaded are gone from the file. If you have already saved, the rest of the data survives only in the original export or the system it came from.

Open the whole file without a row limit

The editor on this site does not have a grid limit: it reads the file from disk as you scroll and draws only what is on screen, so the length of the file does not change how it behaves. Every value is kept exactly as written — leading zeros, long numbers, dates that look like words — because nothing is ever retyped on the way in.

If you only need a count or a check before deciding what to do, the health check reads the whole file and reports rows, columns and what looks off without opening a grid at all.

When Excel is where the file has to end up

Filter, summarise or split first, then hand Excel a file under the limit. Querying with SQL is the direct route: keep the rows you need, group the rest, and download the result as CSV or as an Excel workbook.

A file that is under the row limit but still refuses to open is usually hitting a memory limit instead; the same tools handle that case, since none of them load the whole file into memory to show it.

Questions

Does Excel warn me before dropping the rows?: Once, at open: a small notice that the file was not loaded completely. After that nothing on screen reminds you, which is how the truncated version ends up saved over the original.
If I already saved from Excel, are the rows gone?: From that file, yes — Excel wrote only what it had loaded. Go back to the original export or re-export from the system that produced it; there is nothing in the truncated file to recover from.
Can Google Sheets open it instead?: Sheets has its own ceiling of ten million cells per spreadsheet, so a wide file hits it well before a million rows. It also uploads the file, which may not be acceptable for the data involved.
How do I find out how many rows the file really has?: Run the health check on it. It reads every row and reports the count, along with columns that look inconsistent, without opening the file in a grid.
Will opening it here change anything in the file?: No. The editor keeps every value as the exact text in the file, and the download reproduces the file with only the edits you made — the same quoting and line endings, checked by tests rather than promised.
Is my file uploaded anywhere?: No. The editor reads your file straight from your own disk as you scroll, so a file past Excel’s 1,048,576 rows opens in place rather than being sent somewhere to be split up. The browser is told to refuse any outbound connection while it runs.