"I have to go to the rest room." If you are at a person's house, you might ask, "May I use your bathroom?" If you are in a house, it can be called either the bathroom or rest room. If you are anyplace else, it would be called the rest room, though "bathroom" wouldn't be looked at strangely. If you are among friends, you might say, "I have to go to the john." (Don't ask me how it got that name). Some people might just say "Excuse me, I'll be back in a minute." It would generally be understood where you are going.

Here we say, "Is it OK if I use your toilet?".

fdmaxey is correct! And... there are a lot of slang expressions and euphemisms for this topic. This one is for women only: "Excuse me, I need to powder my nose" It's a little silly and fancy, I don't know any women that wear so much makeup they'd actually do such a thing, but it is a very familiar old-fashioned euphemism that everyone knows. Another general one: to relieve yourself Other slang terms (not so good for polite company) include: to take a leak to take a whiz And then there are some older more obscure ones 'I gotta see a man about a dog.' (= 'I've got to...') Where did this come from??? It turns out the expression originally meant to visit a prostitute but eventually came to mean going to a bathroom. A similar version is 'see a man about a water hole'. By the way, the universal european sign for a bathroom is WC (pronounced 'Vee See' like in german). but americans generally have no idea what this means.

I must agree with Lee from England. It is fun and sometimes useful to know slang in another language. However, I advise everybody I teach slang never to use it. The situations to use slang are very subtle. If you are not extremely competent with a language - and most people who learn in school aren't - slang should never be used. At the very least you can wind up looking awkward and at the worst, you can offend people.

Hello, In order not to be obvious or sound rude (especially around new friends or in formal situations), do not use slang expressions. Try to be polite. "Excuse me, please." "Excuse me, I need (to go to) the toilet." "Excuse me, nature calls." "Excuse me, I'll just be a minute or two." The use of slang is not always appropriate, so try to avoid using it unless you know exactly where, when and how to use it. Also, using slang is not always a good way to impress foreign people. If you are going to take an English test, such as IELTS. Slang is a big NO NO! I hope this helps, Lee.