Tab-separated is what you get pasting straight out of a spreadsheet. Semicolon is what Excel writes in locales that use a comma for the decimal point.
Turn it off to read the CSV as an array of arrays, or to write CSV with no header row — for raw data that never had column names.
Turns "123" into a number, "true" into a boolean and an empty cell into null. Values that would not survive the trip — leading-zero codes, long IDs — are kept as strings.
Nothing here reads as a table.
Convert CSV to JSON and back. Choose the delimiter (comma, tab or semicolon) and whether the first row is a header; commas and line breaks inside quoted fields are read correctly. The parsed table is shown alongside the output.
Formats
CSV / TSV / JSON
Questions
Can it read a CSV with line breaks inside a cell?
Yes. Inside quotes, a line break is part of the value, and the preview table shows it wrapped inside that cell. An unquoted line break is a row separator, which is what the file itself is saying.
It says some rows have a different number of columns
That warning means a row does not match the width of row 1. An unclosed quote, or a delimiter inside an unquoted value, is the usual cause. Look at the preview table to see where the row actually broke. Conversion still runs; missing fields are treated as empty.
How large a file can it take?
Tens of thousands of rows are fine. Because it re-parses as you type, past a few megabytes typing starts to feel sluggish. The preview shows the first 20 rows; copy and save always use everything.
Can it open an .xlsx file?
No — .xlsx is a compressed format of its own. Save as "CSV UTF-8" from Excel, or select the sheet, copy it, and paste it here as tab-separated.
Is the data uploaded?
No. Parsing and conversion happen entirely in the page and are gone when you close the tab, so files that cannot leave the building are fine to use here.