How can I protect an Excel file?

Excel gives you several ways to protect a workbook. You can require a password to open it, a password to change data, and a password for changing the file's structure—adding, deleting, or hiding worksheets.

1. Click File > Save As.
2. Click a location, such as Computer or your My Site web page.
3. Click a folder, such as Documents or one of the folders on your OneDrive, or click Browse.
4. In the Save As dialog box, go to the folder you want to use, then open the Tools list and click General Options.
5. Enter your password, enter it again to confirm, and click OK.

1. make your workbook

2. file>save as>tools>general option>create a password to open and modify>and save 

your file will be protected..................//Abul

• Click Review tab

• Move to Comments group

• Choose Protect Sheet or Protect Workbook by clicking their buttons

In Excel2007,

Go to Review Tab > Protect Sheet

by this you can use password to read only protection of the excel sheet.

we can protect excel file by using protected work sheet

what do you mean by protect [change, read]? you can use password tool:

tools > options > Security tab

OR

tools > protection

open the excel file

in the toolbar click on Review

then you will get the option for 'protect sheet or protect workbook

Depend on how sensitive the document is.

Quick googling found that Words use AES-128 algorithm. While for most this is relatively safe and the most convenient form to password protect, with resources and know how,  AES-128 is crackable using current technology. Zipping the file with password protect will use AES-256 algorithm encryption, this is exponentially harder to crack requiring enormous resources. 

Tools such as Truecrypt could also be considered with more advanced enryption method (http://www.truecrypt.org/docs/hash-algorithms#Y0 ) . While not designed for a single file, this is the safest mean of encryption.

To protect your excel file, on the toolbar click on file then go to info there you will see permissions, there you can encrypt with a password or set restriction.

MS Excel gives you several ways to protect a workbook. You can require a password to open it, a password to edit, and a password for changing the file's structure - adding, deleting, or hiding worksheets.

• Password to Open: File > Info > Protect Workbook > Encrypt with Password.
• Password to Modify: File > Save as > Tools > General Options > Password to Modify.
• Password to enforce Worksheet Read Only: Review > Changes > Protect Sheet.
• Password to avoid hide/unhine or arrange windows: Review > Changes > Protect Workbook.
• Password to Close Cells: make all cells Unlock except you want to close > Review > Changes > Protect Sheet.
• and other options using VBA.