ObjC(Category of NSString)：
- (CGSize)getSizeWithFont:(UIFont *)font constrainedToSize:(CGSize)size{ CGSize resultSize = CGSizeZero; if (self.length <= 0) { return resultSize; } NSMutableParagraphStyle *style = [NSMutableParagraphStyle new]; style.lineBreakMode = NSLineBreakByWordWrapping; resultSize = [self boundingRectWithSize:CGSizeMake(floor(size.width), floor(size.height))//用相对小的 width 去计算 height / 小 heigth 算 width options:(NSStringDrawingUsesFontLeading | NSStringDrawingUsesLineFragmentOrigin) attributes:@{NSFontAttributeName: font, NSParagraphStyleAttributeName: style} context:nil].size; resultSize = CGSizeMake(floor(resultSize.width + 1), floor(resultSize.height + 1));//上面用的小 width（height） 来计算了，这里要 +1 return resultSize; }
Swift(Extension of NSString):
func getSizeWithFont(_ font:UIFont,constrainedToSize size:CGSize) -> CGSize{
if self.length == 0 { return CGSize.zero }
let style = NSMutableParagraphStyle.init()
style.lineBreakMode = .byWordWrapping
let options = NSStringDrawingOptions.usesFontLeading.union(.usesLineFragmentOrigin)
let attributes = [NSFontAttributeName:font,NSParagraphStyleAttributeName:style]
let bound = CGSize.init(width: floor(size.width), height: floor(size.height))
let rect = self.boundingRect(with: bound, options: options, attributes: attributes, context: nil)
let _size = CGSize.init(width: floor(rect.width + 1), height: floor(rect.height + 1))
return _size
}
原文：http://www.cnblogs.com/ficow/p/6418890.html