# 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.*

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