Adjust UILabel height depending on the text

sizeWithFont constrainedToSize:lineBreakMode: is the method to use. An example of how to use it is below:

//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;

In iOS 6 Apple has added a property to UILabel that greatly simplifies dynamic vertical resizing of labels: preferredMaxLayoutWidth.

Using this property in combination with lineBreakMode = NSLineBreakByWordWrapping and sizeToFit method allows easily resize a UILabel instance to the height that accommodates the entire text.

A quote from iOS documentation:

preferredMaxLayoutWidth The preferred maximum width (in points) for a multiline label.

Discussion This property affects the size of the label when layout constraints are applied to it. During layout, if the text extends beyond the width specified by this property, the additional text is flowed to one or more new lines, thereby increasing the height of the label.

A sample:

...
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];
...

UILabel extension based on this answer for Swift 4 and above

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)
    }

}

Can be used like:

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

You can implement TableViewController's (UITableViewCell *)tableView:cellForRowAtIndexPath method in the following way (for example) :

#define CELL_LABEL_TAG 1

- (UITableViewCell *)tableView:(UITableView *)tableView  cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    NSString *text = @"my long text";

    static NSString *MyIdentifier = @"MyIdentifier";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithFrame:CGRectZero  reuseIdentifier:identifier] autorelease];
    }

    CGFloat width = [UIScreen mainScreen].bounds.size.width - 50;
    CGFloat height = [self textHeight:text] + 10;
    CGRect frame = CGRectMake(10.0f, 10.0f, width, height);

    UILabel *cellLabel = [[UILabel alloc] initWithFrame:frame];
    cellLabel.tag = CELL_LABEL_TAG;
    cellLabel.textColor = [UIColor blackColor];
    cellLabel.backgroundColor = [UIColor clearColor];
    cellLabel.textAlignment = UITextAlignmentLeft;
    cellLabel.font = [UIFont systemFontOfSize:12.0f];
    [cell.contentView addSubview:cellLabel];
    [cellLabel release];

    return cell;
}

UILabel *label = (UILabel *)[cell viewWithTag:CELL_LABEL_TAG];
label.text = text;
label.numberOfLines = 0;
[label sizeToFit];
return cell;

Also use NSString's sizeWithFont:constrainedToSize:lineBreakMode: method to compute the text's height.

Thanks for this post. It helped me a great deal. In my case I am also editing the text in a separate view controller. I noticed that when I use:

[cell.contentView addSubview:cellLabel];

in the tableView:cellForRowAtIndexPath: method that the label view was continually rendered over the top of the previous view each time I edited the cell. The text became pixelated, and when something was deleted or changed, the previous version was visible under the new version. Here's how I solved the problem:

if ([[cell.contentView subviews] count] > 0) {
    UIView *test = [[cell.contentView subviews] objectAtIndex:0];
    [test removeFromSuperview];
}
[cell.contentView insertSubview:cellLabel atIndex:0];

No more weird layering. If there is a better way to handle this, Please let me know.

Adding to the above answers:

This can be easily achieved via storyboard.

1. Set constraint for UILabel.(In my case I did top, left and fixed width)
2. Set Number of line to 0 in Attribute Inspector
3. Set Line Break to WordWrap in Attribute Inspector.

Finally, it worked. Thank you guys.

I was not getting it to work because i was trying to resize the label in heightForRowAtIndexPath method:

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath

and (yeah silly me), i was resizing the label to default in cellForRowAtIndexPath method - i was overlooking the code i had written earlier:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath

One line is Chris's answer is wrong.

newFrame.size.height = maximumLabelSize.height;

should be

newFrame.size.height = expectedLabelSize.height;

Other than that, it's the correct solution.

Updates according iOS7

// 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;