# Data validation require unique number

To allow only unique numbers in a given range, you can use data validation with a custom formula based on the AND, ISNUMBER, and COUNTIF functions.

In the example shown, the data validation applied to B5:B9 is:

where ids is the named range B5:B9.

### How this formula works

Data validation rules are triggered when a user adds or changes a cell value.

The AND function takes multiple arguments (logical expressions) and returns TRUE only when all arguments return TRUE. In this case, we need two conditions:

Logical 1 tests if the input is a number using the ISNUMBER function:

ISNUMBER(B5)

The ISNUMBER function returns TRUE when a value is numeric and FALSE if not.

Logical 2 tests checks that the input doesn't already exist in the named range "ids":

COUNTIF(ids,B5)<2

COUNTIF returns a count of the value in B5 inside the named range ids (B5:B9). If the count is less than 2, the logical expression returns TRUE.

If both logical expressions return TRUE, the AND function returns TRUE and validation succeeds:

=AND(TRUE,TRUE) // validation successful

If either logical returns FALSE, data validation fails.

Be aware that numeric input includes dates and times, whole numbers, and decimal values.

*Note: Cell references in data validation formulas are relative to the upper left cell in the range selected when the validation rule is defined, in this case B5.*

## Excel Formula Training

Formulas are the key to getting work done in Excel. In this step-by-step training, you'll learn how to use formulas to manipulate text, work with dates and times, lookup values with VLOOKUP and INDEX & MATCH, count and sum with criteria, dynamically rank values, and create dynamic ranges. You'll also learn how to troubleshoot, trace errors, and fix problems. Start building valuable skills with Excel formulas today. Learn more.