Korea uses 220V at 60Hz with a round two-pin plug of the type common across continental Europe. The US uses 120V at 60Hz with flat blades. Getting this wrong is the most common expensive mistake in importing Korean electronics, and the fix depends entirely on one line of text on the device.
Adapter versus converter versus transformer
A plug adapter changes the shape of the pins and nothing else. A converter or transformer changes the voltage. If the device is dual voltage, a $5 adapter is all you need. If it is 220V only, you need a step-down transformer rated above the device's wattage, which costs $25-80 depending on capacity.
Reading the input rating
Every device or power brick prints an input specification. 100-240V means dual voltage and works anywhere. 220V or 220-240V only means it needs a transformer. AC 220V 60Hz on a Korean appliance is the common case. This is the first thing to check on any electronics listing before you buy.
Sizing a transformer
Buy a transformer rated at least 1.5 times the device wattage to allow headroom, and more for anything with a motor or heating element that draws a surge on startup. A 100W device wants a 200W transformer; a 1,000W hair dryer wants a 2,000W unit, which is heavy and expensive.
Why frequency is not a problem
Korea and the US both run 60Hz, unlike Europe and Japan's western grid. That means devices with motors and clocks behave normally once the voltage is correct, which makes Korea an easier import source electrically than Europe.
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Start a buy-for-me order →Frequently asked
Can I just use a plug adapter on a 220V device?
No. It will receive 120V, and it will either fail to work or be damaged. The voltage has to be converted.
Are travel converters good enough?
Small travel converters are designed for brief use and low wattage. For daily use, buy a proper step-down transformer.
What about USB-powered devices?
Anything charging over USB at 5V works anywhere with the cable you already own. This is why USB devices are the safest Korean electronics to import.