Also think light fabrics for summer, given the heat and high humidity. Synthetics are often better as they dry more quickly than cotton and you’ll probably sweat a lot, but cotton and linen are often more comfortable (when the cotton isn’t wet, or course). Spring and Fall vary, but are generally comfortable in most parts of Japan, so check the climate of the place you’re headed.
A nice raincoat (and some rain pants if you plan on doing a lot of biking) is a good idea for the rainy season and some of the sporadic summer storms, but it is possible to get those things here, from high-end, breathable, waterproof, brand-name gear to the cheap rain outfits that most convenience stores sell.
Aside from climate, the rest depends on the type of job you’ll be doing, and what size you wear.
So here’s where we get to sizes. Everyone has different body types, so your experience in Japan may vary depending on this (for example, two women could wear a size 6 in the US, but one may need long pants and the other, short/petite - or one needs more room in the waist or hips, and the other doesn’t).
For reference, I’m a US size 4 or small in most US brands, and I can usually wear Japanese size medium (sometimes I go with large though if I want more room, such as with jackets). My one (minor) gripe is that long shirt sleeves tend to be shorter, as I prefer my sleeves to go just past my wrists. Not a big issue though. I also have an issue with pants usually being a bit too short, so I usually buy up in pant sizes here. I’m 5’6” (167cm), but have a short torso and longer legs. I know some people have the opposite problem, but it’s easy to have pants hemmed up.
I’ve also found that western brands (such as Gap, or outdoor brands, such as Patagonia, Columbia, etc.) typically sell their items in the same western sizes, so you may find that if you wear a large at Gap in the US, you’ll most likely wear a large at Gap in Japan (this is assuming you even shop at Gap though, but just as an example).
A lot of people like Uniqlo (see 6 reasons to shop at Uniqlo) for its cheap basics, and women’s sizes go up to L, sometimes XL or XXL online. Men’s usually go up to XL (91 or so in numbered pants). My husband usually wears medium/large in the US, and has no problems fitting into large in Japan, but he has also noted the long shirt sleeves are a bit shorter. He hasn’t had a problem with pants at all (he’s 5’11”/178cm tall, and usually wears 32/32 US size in pants), and typically wears 82/85 numbered pants in Japan.
So if you’re a women’s large size US or men’s XL/XXL and up (please make appropriate conversions for your country’s sizes), you may have more difficulty finding clothes that fit properly here. However, would love to hear your experience - and if you have specific US/UK/etc. size examples you’ve found to be difficult to find in Japan, let us know.
I’ve yet to find many plus sizes around Japan, though occasionally I’ve seen some via Uniqlo’s website and some department stores or other stores do carry them. So, if anyone knows of any other resources on that, feel free to share in the comments below and I’ll add them in.
John had this to share regarding plus size clothing in Tokyo:
There is a shop that sells dress shirts, polos, and suits for plus size men (actually all size men). It's called Sakazen and there's quite a few around Tokyo. The stores generally have like 5 or 6 floors. The prices are fairly cheap, but some floors have more "exclusive" brands which are slightly overpriced. It's worth a visit
How about shoes?
If you’re a woman and wear above a 25 (Japan size), you will likely have some difficulty finding shoes. Amazon Japan does have some options up through 29 (very small selection), and I know some other select stores occasionally do as well, but in my experience they are fewer and far between (keep in mind larger sizes are easier to find in large cities). I typically wear a 25 in Japan (about 8.5-9 US) and though I’ve found some shoes in my size, they don’t always fit. Sometimes shoes designated with a “large” size work, but depends on the type of shoe. Not impossible, but it’s easier for me to find shoes that fit online. Women's conversion chart.
And for the men, conversion chart. My husband says Japan size 26-27 is about the size that borders difficulty, though we’ve had no problems finding him athletic shoes that fit (he usually wears 27.5-28 Japan size [US 10.5-11]) if we visit sports stores in the bigger cities. Casual and business shoes will vary depending on your location, the brand, store, etc.
