UILabel+Fit.h

@interface UILabel (Fit)
- (void)fitFontSize
@end

UILabel+Fit.m

@implementation UILabel (Fit)

- (void)fitFontSize
{
    UIFont *font = self.font;
    NSInteger maxFontSize = font.pointSize;
    CGFloat width = CGRectGetWidth(self.frame);
    CGFloat height = CGRectGetHeight(self.frame);
    CGSize constraintSize = CGSizeMake(width, CGFLOAT_MAX);
    
    for (NSInteger i = maxFontSize; i >= self.minimumFontSize; i--) {
        font = [font fontWithSize:i];

        CGSize expectedLabelSize = [self.text sizeWithFont:font
                                         constrainedToSize:constraintSize];

        if (expectedLabelSize.height <= height)
            break;
    }

    self.font = font;
}

@end