RadNumericUpDown uses String.Format() to format its string depending on the current culture and number format. The behavior of String.Format() is to multiply the value by 100 when it formats using percentage number format. For example the result of:
will be "500.00 %" (the decimal separator depends on your culture settings).
To workaround this behavior you could use ValueFormat="Numeric" and set CustomUnit="%" - this way the control will behave as with regular numbers, but will display "%" at the end.
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