您可以在设计时在Storyboard/XIB中这样做，而不是通过编程来完成。

在属性检查器中将UIlabel的行数属性设置为0。 然后根据需要设置宽度约束/(或)前导和后导约束。 然后用最小值设置高度约束。最后选择你添加的高度约束，在大小检查器属性检查器旁边，改变高度约束的关系从等于-大于。

此方法适用于iOS 6和7

- (float)heightForLabelSize:(CGSize)maximumLabelSize  Font:(UIFont *)font String:(NSString*)string {

if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 7) {
    NSDictionary *stringAttributes = [NSDictionary dictionaryWithObject:font forKey: NSFontAttributeName];

    CGSize adjustedLabelSize = [string maximumLabelSize
                                                                  options:NSStringDrawingTruncatesLastVisibleLine|NSStringDrawingUsesLineFragmentOrigin
                                                               attributes:stringAttributes context:nil].size;
    return adjustedLabelSize.height;
}
else {
    CGSize adjustedLabelSize = [string sizeWithFont:font constrainedToSize:maximumLabelSize lineBreakMode:NSLineBreakByWordWrapping];

    return adjustedLabelSize.height;
}

}

谢谢大家的帮助，这是我尝试的代码，这是为我工作

   UILabel *instructions = [[UILabel alloc]initWithFrame:CGRectMake(10, 225, 300, 180)];
   NSString *text = @"First take clear picture and then try to zoom in to fit the ";
   instructions.text = text;
   instructions.textAlignment = UITextAlignmentCenter;
   instructions.lineBreakMode = NSLineBreakByWordWrapping;
   [instructions setTextColor:[UIColor grayColor]];

   CGSize expectedLabelSize = [text sizeWithFont:instructions.font 
                                constrainedToSize:instructions.frame.size
                                    lineBreakMode:UILineBreakModeWordWrap];

    CGRect newFrame = instructions.frame;
    newFrame.size.height = expectedLabelSize.height;
    instructions.frame = newFrame;
    instructions.numberOfLines = 0;
    [instructions sizeToFit];
    [self addSubview:instructions];

你也可以把它作为一种方法。@Pyjamasam是非常正确的，所以我只是在创建它的方法。它可能对其他人有帮助

-(CGRect)setDynamicHeightForLabel:(UILabel*)_lbl andMaxWidth:(float)_width{
    CGSize maximumLabelSize = CGSizeMake(_width, FLT_MAX);

    CGSize expectedLabelSize = [_lbl.text sizeWithFont:_lbl.font constrainedToSize:maximumLabelSize lineBreakMode:_lbl.lineBreakMode];

    //adjust the label the the new height.
    CGRect newFrame = _lbl.frame;
    newFrame.size.height = expectedLabelSize.height;
    return newFrame;
}

就像这样

label.frame = [self setDynamicHeightForLabel:label andMaxWidth:300.0];

基于Swift 4及以上答案的UILabel扩展

extension UILabel {

    func retrieveTextHeight () -> CGFloat {
        let attributedText = NSAttributedString(string: self.text!, attributes: [NSFontAttributeName:self.font])

        let rect = attributedText.boundingRect(with: CGSize(width: self.frame.size.width, height: CGFloat.greatestFiniteMagnitude), options: .usesLineFragmentOrigin, context: nil)

        return ceil(rect.size.height)
    }

}

可以这样使用:

self.labelHeightConstraint.constant = self.label.retrieveTextHeight()

由于sizeWithFont已弃用，我使用这个代替。

这个得到标签特定属性。

-(CGFloat)heightForLabel:(UILabel *)label withText:(NSString *)text{

    NSAttributedString *attributedText = [[NSAttributedString alloc] initWithString:text attributes:@{NSFontAttributeName:label.font}];
    CGRect rect = [attributedText boundingRectWithSize:(CGSize){label.frame.size.width, CGFLOAT_MAX}
                                           options:NSStringDrawingUsesLineFragmentOrigin
                                           context:nil];

    return ceil(rect.size.height);
}