File Formats: .xlsx, .xlsm, .csv, .xlsb

.xlsx — the default

It is actually a zip file. Try it: copy any .xlsx, change the extension to .zip, and open it — you'll find XML files inside. That's why Python (openpyxl) and other tools can read them.

Formulas, formatting, charts, pivot tables all save here. Macros do not.

.xlsm — for macros

If you write VBA code and save as .xlsx, Excel will warn you and then strip the code out. Macros require .xlsm.

The downside: many companies and email systems block .xlsm attachments for security reasons. Check before sending one to a client.

.csv — the lingua franca of data

Pure plain text. One line per row, columns separated by commas.

Name,City,Amount
Rahul,Delhi,5000
Priya,Mumbai,7200

What CSV does NOT save:

• Formulas (only their results survive)
• Formatting, colours, fonts
• Charts, pivot tables, images
• Multiple sheets — only the active sheet survives

That last point is the dangerous one. Save a five-sheet file as CSV and four sheets are gone, with only a small warning that most people dismiss by hitting Enter.

The real CSV trap: leading zeros don't survive. A pincode of 110005 is fine, but 0110005 opens as 110005. The same happens to phone numbers, employee IDs and account numbers. So never open a CSV by double-clicking it — import it via Data → From Text/CSV, where you can set the column type to Text.

.xlsb — for large files

A binary format. Same features as .xlsx, but roughly half the file size and noticeably faster to open and save. The drawback is that some tools — Python libraries, Google Sheets — can't read it. Use it only when a file is huge and will stay inside Excel.

The decision, in one block

Has macros?            → .xlsm
Sending to another system? → .csv
File over 100MB?       → .xlsb
Everything else        → .xlsx