I have issues calculating the accurate size of a NSString displayed in a UITextField.

My goal is to update the textfield frame size according to the string size programmatically (without using sizeToFit). I am using the sizeWithFont function.

-(void)resizeTextFieldAccordingToText:(NSString*)textFieldString {

    CGPoint originalCenter = self.textField.center;

    UIFont* currentFont = [textField font];
    CGSize newSize = [textFieldString sizeWithFont:currentFont];

    //Same incorrect results with the extended version of sizeWithFont, e.g.
    //[textFieldString sizeWithFont:currentFont constrainedToSize:CGSizeMake(300.0, 100.0) lineBreakMode:NSLineBreakByClipping];

    [self.textField setFrame:(CGRectMake(self.textField.frame.origin.x, self.textField.frame.origin.y, newSize.width, newSize.height))];
    [self.textField setCenter:originalCenter];

} 

Problem: While this return correct size results at first its becomes more and more unprecise by adding characters therefore finally starts clipping the string (as seen in the right screenshot).

How do I get the accurate size of the textField string for correctly adjusting its size?