How to get the width of an NSString?

Here's a relatively simple approach. Just create an NSAttributedString with the appropriate font and ask for its size:

- (CGFloat)widthOfString:(NSString *)string withFont:(NSFont *)font {
     NSDictionary *attributes = [NSDictionary dictionaryWithObjectsAndKeys:font, NSFontAttributeName, nil];
     return [[[NSAttributedString alloc] initWithString:string attributes:attributes] size].width;
 }

Using the UILabel's attributed string:

- (CGSize)getStringSizeWithText:(NSString *)string font:(UIFont *)font{

    UILabel *label = [[UILabel alloc] init];
    label.text = string;
    label.font = font;

    return label.attributedText.size;
}

Send the string a sizeWithAttributes: message, passing a dictionary containing the attributes with which you want to measure the string.

UIFont * font ＝ [UIFont systemFontOfSize:15];

CGSize stringSize = [aString sizeWithFont:font]; 

CGFloat width = stringSize.width;