One sheet of values
Spreadsheets fail in conversion at the boring points: a second sheet that nobody exported, a date that Excel rewrote, a leading zero that vanished, a comma inside a name that split one column into two.
CSV is one sheet of values. JSON can nest, and a flat CSV cannot represent that without leftover columns. After any of these tools, open the file in a text editor before you let Excel guess the types.
If you already know the job, start with Excel to CSV or CSV to Excel.
Every format on the site is listed from the homepage.
Check dates before you trust the import
Open the tool that names your two formats and read what the download will contain before you delete the original. If a result does not match that description, tell us.
Values, then the boring checks
Excel, CSV, TSV, JSON, and ODS tools move tables without retyping. They do not move a dashboard. Formulas become values when they leave the workbook. Charts and macros stay behind. CSV is one sheet. A comma inside a cell must be quoted or it becomes two columns. TSV avoids that comma problem and still breaks if a cell contains a tab. JSON works when you have a list of similar objects. Nested lists of unknown length will not become one clean sheet.
XLS is the pre-2007 workbook. Use it only when a program refuses XLSX, and check the last row, because the old format can drop data. ODS is for LibreOffice. Open it there before you send it. After any export, look at dates and leading zeros. Excel guesses types when it opens a text file. The password-protected workbook will not open in the browser. Keep the file you calculated in.
Count the rows before and after. A silent drop is worse than an error message. Check one date, one identifier with a leading zero, and the last sheet name. If any of those changed, fix the import settings in Excel or send the workbook instead of the CSV. JSON is for a developer who will read the structure. It is a poor attachment for a colleague who only has Excel. Say which sheet was exported in the file name so the next person does not assume the hidden tabs came along.
Row count, dates, and the last sheet
Write down the row count and the sheet names before you export. Afterward, those two notes tell you whether the file is complete. A CSV will not contain sheet two. A date that flipped day and month will still look like a date. An identifier that lost its leading zero will still look like a number. Check those three on purpose. Excel will not warn you.
Send the workbook when the recipient will edit formulas. Send CSV or TSV when a database will import values. Send JSON when a program will parse structure. Those are different gifts. Name the file with the sheet you exported so nobody assumes the hidden tabs came along. Password-protected workbooks need to be opened in Excel first.
Freeze the header row in the workbook before you export so you know which row is titles. The CSV will not freeze anything. It will just put that row first if it was first. If your sheet has a title above the headers, delete that title row from the copy you export, or the database will treat the title as a column name. Do that on a copy. Leave the formatted workbook alone.