UILabel+Resize.m

- (CGFloat)heightThatFits {
    if(!self.text || [self.text isEqualToString:@""])
        return 0;
    
    if([self respondsToSelector:@selector(attributedText)]) {
        CGSize maxSize = (CGSize){ self.frame.size.width, MAXFLOAT };
        CGRect frame = [self.attributedText boundingRectWithSize:maxSize
                                                         options:(NSStringDrawingUsesLineFragmentOrigin|
                                                                  NSStringDrawingUsesFontLeading)
                                                         context:nil];
        
        return ceilf(frame.size.height);
    } else {
        #if __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_6_0
        #warning No more iOS 5? Remove me!
        #endif
        
        CGSize maxSize = (CGSize){ self.frame.size.width, MAXFLOAT };
        CGSize size = [self.text sizeWithFont:self.font
                            constrainedToSize:maxSize
                                lineBreakMode:self.lineBreakMode];
        
        return ceilf(size.height);
    }
}

UITextView+Resize.m

- (CGFloat)heightThatFits {
    if(!self.text || [self.text isEqualToString:@""])
        return 0;
    
    if([self respondsToSelector:@selector(textContainer)]) {
        [self.layoutManager ensureLayoutForTextContainer:self.textContainer];
        [self layoutIfNeeded];
        
        CGFloat height = [self.layoutManager usedRectForTextContainer:self.textContainer].size.height;
        height += self.textContainerInset.top + self.textContainerInset.bottom;
        
        return ceilf(height);
    } else {
        #if __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_7_0
        #warning No more iOS 6? Remove me!
        #endif
        
        CGFloat height = self.contentSize.height;
        height += (self.contentInset.top + self.contentInset.bottom);
        
        return ceilf(height);
    }
}

#pragma mark -

- (CGFloat)maxHeight {
    return 8192;
}

- (CGFloat)closestMaxHeight {
    __block CGFloat height = self.maxHeight;
    
    CGPoint point = CGPointMake(0, self.maxHeight);
    NSUInteger glyphIndex = [self.layoutManager glyphIndexForPoint:point inTextContainer:self.textContainer];
    NSUInteger charIndex = [self.layoutManager characterIndexForGlyphAtIndex:glyphIndex];
    
    NSRange range = NSMakeRange(0, charIndex);
    NSStringEnumerationOptions options = (NSStringEnumerationByParagraphs|NSStringEnumerationReverse);
    NSStringEnumerationBlock block = ^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
        *stop = YES;
        
        NSUInteger glyphIndex = [self.layoutManager glyphIndexForCharacterAtIndex:substringRange.location];
        CGRect lineRect = [self.layoutManager lineFragmentRectForGlyphAtIndex:glyphIndex effectiveRange:NULL];
        
        height = floorf(lineRect.origin.y);
    };
    
    [self.text enumerateSubstringsInRange:range options:options usingBlock:block];
    
    return ceilf(height);
}