The bottle says, “SPF 50,” but we have to take into account that France is a Metric country. SPF, of course, is a unit-less number. Metric unit-less scalar numbers are worth roughly 1.4 times their Imperial counterparts. Now you might be tempted to cube this number to get the conversion factor because sunscreen is a liquid, three-dimensional. But, in fact, the number refers to the light absorption properties of the skin. The skin's surface-area is a two-dimensional quantity (measured in square-meters or square-feet), but its exposure of light is measured per-unit of area, so the conversion factor is the inverse-square of 1.4, which is a little more than a half. So this sunscreen is about SPF 25, by American standards.

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.