How to figure out the font size of a UILabel when -adjustsFontSizeToFitWidth is set to YES?

I'm not sure if this is entirely accurate, but it should be pretty close, hopefully. It may not take truncated strings into account, or the height of the label, but that's something you might be able to do manually.

The method

- (CGSize)sizeWithFont:(UIFont *)font minFontSize:(CGFloat)minFontSize actualFontSize:(CGFloat *)actualFontSize forWidth:(CGFloat)width lineBreakMode:(UILineBreakMode)lineBreakMode

will return the text size, and notice that it also has a reference parameter for the actual font size used.

In case anybody still needs the answer. In iOS9 you can use boundingRectWithSize:options:context: to calculate actual font size. Note that context.minimumScaleFactor should not be 0.0 for scaling to work.

- (CGFloat)adjustedFontSizeForLabel:(UILabel *)label {
    NSMutableAttributedString *text = [[NSMutableAttributedString alloc] initWithAttributedString:label.attributedText];
    [text setAttributes:@{NSFontAttributeName:label.font} range:NSMakeRange(0, text.length)];

    NSStringDrawingContext *context = [NSStringDrawingContext new];
    context.minimumScaleFactor = label.minimumScaleFactor;
    [text boundingRectWithSize:label.frame.size options:NSStringDrawingUsesLineFragmentOrigin context:context];
    CGFloat adjustedFontSize = label.font.pointSize * context.actualScaleFactor;

    return adjustedFontSize;
}

For one-line UILabel works fine this simple solution:

//myLabel - initial label

UILabel *fullSizeLabel = [UILabel new];
fullSizeLabel.font = myLabel.font;
fullSizeLabel.text = myLabel.text;
[fullSizeLabel sizeToFit];

CGFloat actualFontSize = myLabel.font.pointSize * (myLabel.bounds.size.width / fullSizeLabel.bounds.size.width);

//correct, if new font size bigger than initial
actualFontSize = actualFontSize < myLabel.font.pointSize ? actualFontSize : myLabel.font.pointSize;