There are two main ways to apply data validation to a range of cells at once in Excel:

Method 1: Select the Range Before Applying Validation

1. Select the cell range: Click and drag your mouse to highlight all the cells where you want to apply the same data validation rule.
2. Go to Data Validation: Navigate to the "Data" tab on the Excel ribbon. In the "Data Tools" group, click "Data Validation."
3. Set your validation criteria: In the Data Validation window, define the type of allowed data (e.g., List, Number, Text), set any specific limits or formulas, and add informative messages if desired (Input Message & Error Alert tabs).
4. Click OK: Once you've configured the validation rule, click "OK" to apply it to all the selected cells in the range.

Method 2: Apply Validation to One Cell, Then Copy Formatting

1. Set validation on a single cell: Choose one cell within the range you want to validate. Apply the desired data validation rule following steps 2 and 3 from method 1.
2. Copy the formatting: Right-click on the cell with the validation rule and select "Copy" from the context menu.
3. Select the entire range: Highlight all the cells where you want to apply the validation.
4. Paste formatting: Right-click anywhere within the selected range and choose "Paste Special" from the context menu. In the "Paste Special" window, select "Formats" and click "OK."

This method copies only the cell formatting, which includes the data validation rule.

Additional Tips:

* You can check the "Apply these changes to all other cells with the same settings" box within the Data Validation window if you have other cells with identical formatting that you want to validate simultaneously.
* Remember that data validation only affects how users enter data directly into cells. It won't prevent invalid entries if data is pasted or copied from another source.