Because of the need to convert uniquely Japanese number concepts, a surprisingly valuable tool for the Japanese to English translator is a humble calculator.
For example, the floor area of a hotel room or apartment is often given in 畳 (jō) or tatami mats, the size for which actually varies from place to place, but for rough and ready purposes can be considered to be 1.653 meters squared. Meanwhile, the area of a temple garden might be given in 坪 (tsubo), which is considered equal to two tatami mats or 3.306 meters squared.
Of course, how one handles such measurements also depends on the context of the material you are working on. In a tourism brochure, one would probably convert these straight through to meters (or feet, etc., if exclusively for a US audience), cutting off any needlessly long decimal points or even rounding to the nearest thousand, etc., if a particularly large number. Meanwhile if you were working on a historical novel, you might purposefully keep things in jō and tsubo, possibly with an explanatory footnote the first time they were mentioned, to add an exotic flair.
Another numerical concept requiring frequent translation is the Japanese imperial year system. Traditionally, in Japan, years are dated based on the number of years which have passed since the emperor reigning during a given time period took the throne. This system is still widely used in official documents today and also frequently pops up in all manner of other materials. Interestingly, this is only used for years – days and months are the same as for the Gregorian calendar used in the vast majority of the world (so at least those don’t require any arithmetic).
Some people probably get this system right away, but I struggled with it for quite awhile and always had to look up online converters to switch Japanese years into Gregorian years, until I finally figured out the trick (and felt pretty stupid when I did). Because the year in which an imperial reign starts is counted as year 1, to convert a Japanese imperial year to a Gregorian year, all you have to do is subtract one from the Japanese year and then add that to the Gregorian year the particular emperor’s reign started. I know that sounds a little tricky, but if you’re working with modern and not historical materials, generally you only need to remember the starts of four consecutive imperial eras – 明治/Meiji (1868), 大正/Taisho (1912), 昭和/Showa (1926), and 平成/Heisei (1989). So, for example, 平成13 = 13 -1 + 1989 = 12 + 1989 = 2001.
Like with our jō and tsubo, example, it depends on the context, but in most situations one would probably avoid noting the era name and just convert the date to standard Gregorian. Of course, the date format itself (mm/dd/yy? dd/mm/yy? yyyy/mm/dd?) is a whole other can of worms. And next year, in 2019, the current emperor will be stepping down, necessitating the creation of a new era name and start year for translators to remember. Better keep that calculator handy!