sizeWithFont constrainedToSize:lineBreakMode:是使用的方法。如何使用它的例子如下:

//Calculate the expected size based on the font and linebreak mode of your label
// FLT_MAX here simply means no constraint in height
CGSize maximumLabelSize = CGSizeMake(296, FLT_MAX);

CGSize expectedLabelSize = [yourString sizeWithFont:yourLabel.font constrainedToSize:maximumLabelSize lineBreakMode:yourLabel.lineBreakMode];   

//adjust the label the the new height.
CGRect newFrame = yourLabel.frame;
newFrame.size.height = expectedLabelSize.height;
yourLabel.frame = newFrame;

根据iOS7更新

// If description are available for protocol
protocolDescriptionLabel.text = [dataDictionary objectForKey:@"description"];
[protocolDescriptionLabel sizeToFit];
[protocolDescriptionLabel setLineBreakMode:NSLineBreakByWordWrapping];

CGSize expectedLabelSize = [protocolDescriptionLabel
               textRectForBounds:protocolDescriptionLabel.frame
               limitedToNumberOfLines:protocolDescriptionLabel.numberOfLines].size;
NSLog(@"expectedLabelSize %f", expectedLabelSize.height);

//adjust the label the the new height.
CGRect newFrame = protocolDescriptionLabel.frame;
newFrame.size.height = expectedLabelSize.height;
protocolDescriptionLabel.frame = newFrame;

这是使用Objective-c获取UILabel Height的一行代码:

labelObj.numberOfLines = 0;
CGSize neededSize = [labelObj sizeThatFits:CGSizeMake(screenWidth, CGFLOAT_MAX)];

使用.height你会得到标签的高度，如下所示:

neededSize.height

当自动布局被启用时，调整大小不工作:)

问题是，没有提到的函数是可靠的，一些字符串和字体将返回不正确的高度值。尤其是带属性的文本。

唯一可行的解决方案是:https://stackoverflow.com/a/4214978/699944，重点是使用CoreText手动计算每一行的高度，以获得正确的大小。没有其他已知的方法可以做到这一点。

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;
}

}