Do I need a voltage converter for Japan, for MacBooks or USB devices? Japan uses 100V; I will be in parts of the country using both 50Hz and 60Hz. Will I need a voltage converter for a) a MacBook power supply, b) a standard AC-to-USB plug? I live in the U.S.
The MacBook power supply is rated for 50-60hz and 100-240V. The 5W USB also falls within these specs.

You can confirm your own devices by looking at the power brick and reading the input section. (See image below.)

The non-polarized, ungrounded, two prong US plug style works in Japanese outlets, so an Apple 5W USB charger will fit without an adapter. The MacBook charger will work if you use the two prong attachment instead of the grounded, three prong extension.

You do not need a voltage, frequency, or plug-shape adaptor in Japan with a US MacBook power supply and iPhone/iPad USB power supply.



Note that while the standard US two prong plug fits in a Japanese outlet, you still have to make sure that the device can use 100V, 50/60Hz power. Different devices and brands have different requirements. Be careful to confirm the input voltage and frequency of your power supply. Using a power supply on a power system that is out of range can cause damage to your device.