Defining field validation
You can select validation options to ensure that data is entered into a field correctly. When validation options are selected, FileMaker Pro displays a message if you enter data incorrectly. For example, you can set an option to require that users enter a value in a field.
Important  To avoid confusion when using dates in FileMaker Pro, set field validation options to make sure dates are always entered with four-digit years. For more information on how FileMaker Pro handles dates with two-digit years, see Conversion of dates with two-digit years.
To choose field validation options:
With the database open, choose File menu > Manage > Database.
Note  To set options for fields in Table View, right-click the column heading for the field, choose Field > Field Options from the shortcut menu, then skip to step 8.
Click Options (or double-click the field name).
For Validate data in this field, select Always for ongoing field validation, or Only during data entry to limit validation to instances when data is being entered. Check Allow user to override if you want the user to override validation warnings.
Strict data type, then choose Numeric Only from the list
Strict data type, then choose 4-Digit Year Date from the list
Strict data type, then choose Time of Day from the list
Member of value list, then choose a value list. (Choose Manage Value Lists to create a list.)
In range, then type the beginning and ending values
Validated by calculation (or click Specify), then enter a calculation. See Defining calculation fields for information about the dialog box.
Select or clear Validate only if field has been modified, then click OK.
Maximum number of characters, then type the maximum number of characters allowed
Maximum number of kilobytes, then enter the number of kilobytes. This setting limits the size of a file that’s embedded in a container field or stored externally. This setting has no effect on files that are stored by reference in a container field.
To display a custom message if validation fails, select Display custom message if validation fails. Type a message (up to 255 characters).
Click OK to close the Options for Field dialog box, or click another tab to set additional field options.
Field and validation values must be identical to successfully validate the contents of a field. If the Validation by calculation option is used, the calculation results must exactly match the contents of the field being evaluated. By default, the FileMaker Pro calculation engine calculates to 16 digits of precision to the right of the decimal. If you are validating fields by calculation that return fractional results, you can use the Round function to limit calculation results to an expected number of digits.
Related topics 
Working with formulas and functions
Entering and changing data in fields
Committing data in records
Defining value lists