准确计算CoreText高度的方法：

- (int)getAttributedStringHeightWithString:(NSAttributedString *)  string  WidthValue:(int) width  
{  
   int total_height = 0;  
     
   CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)string);    //string 为要计算高度的NSAttributedString  
   CGRect drawingRect = CGRectMake(0, 0, width, 1000);  //这里的高要设置足够大  
   CGMutablePathRef path = CGPathCreateMutable();  
   CGPathAddRect(path, NULL, drawingRect);  
   CTFrameRef textFrame = CTFramesetterCreateFrame(framesetter,CFRangeMake(0,0), path, NULL);  
   CGPathRelease(path);  
   CFRelease(framesetter);  
     
   NSArray *linesArray = (NSArray *) CTFrameGetLines(textFrame);  
     
   CGPoint origins[[linesArray count]];  
   CTFrameGetLineOrigins(textFrame, CFRangeMake(0, 0), origins);  
     
   int line_y = (int) origins[[linesArray count] -1].y;  //最后一行line的原点y坐标  
     
   CGFloat ascent;  
   CGFloat descent;  
   CGFloat leading;  
     
   CTLineRef line = (CTLineRef) [linesArray objectAtIndex:[linesArray count]-1];  
   CTLineGetTypographicBounds(line, &ascent, &descent, &leading);  
     
   total_height = 1000 - line_y + (int) descent +1;    //+1为了纠正descent转换成int小数点后舍去的值  
     
   CFRelease(textFrame);  
     
   return total_height;  
     
}  

来自：http://blog.csdn.net/iunion/article/details/7925951