我的代码:

UILabel *label      = [[UILabel alloc] init];
label.numberOfLines = 0;
label.lineBreakMode = NSLineBreakByWordWrapping;
label.text          = text;
label.textAlignment = NSTextAlignmentCenter;
label.font          = [UIFont fontWithName:_bodyTextFontFamily size:_bodyFontSize];

CGSize size = [label sizeThatFits:CGSizeMake(width, MAXFLOAT)];


float height        = size.height;
label.frame         = CGRectMake(x, y, width, height);

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);
}

在iOS 6中，苹果为UILabel添加了一个属性，极大地简化了标签的动态垂直大小调整:preferredmaxlayoutidth。

将此属性与lineBreakMode = nslinebreakbywordwrapped和sizeToFit方法结合使用，可以轻松地调整UILabel实例的大小，以容纳整个文本的高度。

引用iOS文档:

preferredMaxLayoutWidth 多行标签的首选最大宽度(以点为单位)。 讨论 当对标签应用布局约束时，此属性将影响标签的大小。在布局过程中，如果文本扩展到超出此属性指定的宽度，则附加文本将流动到一个或多个新行，从而增加标签的高度。

一个示例:

...
UILabel *status = [[UILabel alloc] init];
status.lineBreakMode = NSLineBreakByWordWrapping;
status.numberOfLines = 5; // limits to 5 lines; use 0 for unlimited.

[self addSubview:status]; // self here is the parent view

status.preferredMaxLayoutWidth = self.frame.size.width; // assumes the parent view has its frame already set.

status.text = @"Some quite lengthy message may go here…";
[status sizeToFit];
[status setNeedsDisplay];
...

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;

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];

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;