If you think that's bad, it gets much more confusing in Britain, which is mostly on the Metric system except for grumpy old people and in matters of alcohol. Grumpy old people don't buy a lot of sunscreen, but if you've had a couple pints (which are of course different from American pints), you might have to take this into consideration. In this case there may be a distinction based on the length of the foot of the reigning monarch. In the 19th Century, as Dickens writes in A Tale of Two Cities, the King of England had a square jaw, so the ratio of his foot size to the standard foot was introduced squared. This was a serious inconvenience when buying sunscreen in these days, when people really had a whole lot else to worry about. The resulting epidemic of sunburn precipitated the tragic plot of Dickens' great novel, which is remembered as a rallying cry in the movement to adopt the Metric system in Britain. Ironically, when this finally happened, sunscreen applied while drunk retained this annoying conversion. Fortunately the monarch from then through today, Elizabeth II, has more of a round jaw, so though her feet are shorter than a foot, they just round up to a foot and the conversion is moot. So Americans and Europeans alike can consider SPF numbers in Britain exactly as they're accustomed to in their own countries.
But if you're from Australia you have to turn the bottle upside-down before reading.