As we are all adults here, how do I politely indicate that I need the toilet?

The joys of English mean there’s no actual word as such for the room in question. Everything is a euphemism.

a mild or indirect word or expression substituted for one considered to be too harsh or blunt when referring to something unpleasant or embarrassing.

Which means that just about anything on the list is valid.

Americans seem to like Restroom and Bathroom. In the rest of the English speaking world you’re likely to be sent to the wrong place in either case as they might think you want a rest or a bath.

In England, in quiet mixed company a question of Where’s the um..? will usually get you to the right place. Toilet, lavatory or loo are quite acceptable. Little boys room (if gender appropriate) also quite acceptable in a social setting though might raise a laugh. John, Head, Karzi, Bog, Lav are situational and you’ll rarely be in the situation. Crapper is actually well understood but almost never used and if you’re among people you know you could just stand up and state that you need to take a slash, which way should you go.

Many years ago, a coworker (new at that time) mentioned to me in a matter-of-fact way that he intends to visit the boys’ corner.
I was confused and asked him what he meant, to which he translated it as “toilet”. I was amused with the realisation!
It may not be a polite way of inquiring with the other gender, though!

I’m Australian. We like directness. Among friends we can be very blunt: “Where’s the dunny?”

Among foreigners we tone it down to merely informational and if we think the other person might be offended we use a more discreet low tone and start with a warning about impending directness: “I don’t know the local polite form so I’m going to be direct. Where is the toilet, men’s room or whatever you call it here?”

You won’t offend and you will get two important pieces of information.

If your looking for a universal way that fits all circumstances, your best bet is going to be to ask someone. Your going to be the guest of someone. Even in a store you have the employees to help. So find someone that looks like they know the place, or better yet, invited you, and quietly, discreetly, let them know what your looking for. Then listen to the words they use, and don’t be afraid to use them too.

For example, ask the person who invited you to the dinner party, quietly, “I don’t really know how to ask properly, but I need to pee.” They will respond based on their culture. If your talking with just one person and your quite, then it will be hard for them to be offended.

If your not comfortable with that, then ask for the “Lavatory”. It’s not exactly correct, but almost every English native speaker will be able to know what your after without making a scene.

As another general rule, there is no reason to state why you need that room. Again if your talking to one person, and that person is your host, or guide, then your probably fine. If your speaking to a group or something, then I would, generally, avoid saying why.

After a few days in an area, your should get the hang of it. When in doubt, ask the service staff. There the least likely to be offended by a simple, quite, question.

The politest thing to do is to not draw attention to the fact that you need the toilet.

Don’t indicate to all and sundry that you need the toilet.

If you need to know where the toilet is, ask where is the toilet/restroom/bathroom/lavatory, please?

If you are desperate to go during a meeting or conversation, quietly excuse yourself at an opportune moment and just say I’ll be back in a minute. Everyone doesn’t need to know where you are going and what you are doing.

Avoid slang terms.

People know that foreigners sometimes speak a little strangely and use unconventional words occasionally.

Use common sense propriety and nobody will think any the less of you.

It pretty much depends on which part of the world you are in.

Your choice of words could be limited by their command of English and their history (e.g. an American colony in the following example).

I remember arriving to Ninoy Aquino International Airport in Manila, and asking to a member of the airport crew where could I go to the loo, wc, gents, bathroom…and so on, until I hit on the toilet word. “oh, you want to go to the toilet…”

Actually asking for the toilets is not the difficult part, as the habit is pretty universal. The tricky part is the variations in cultures and habits about the cleaning part…

In the United States, the more polite terms I hear are usually either the restroom or the ladies’ room/men’s room (always including room). Washroom sounds Canadian to me. British terminology (such as loo) may well be understood but would sound odd.

So, I might ask: “Where’s the restroom?” If I already knew, I’d most likely excuse myself without explanation.

BEWARE: In the United States, using the word toilet to mean anything other than the actual porcelain fixture sounds shocking or rude. The difference between British and American English in this regard is quite pronounced. The first time I visited the UK, I was quite surprised to see signs which read “Toilet.” You would never see such in the US.

Also, I’ve encountered people in the UK who don’t understand the word restroom, which is usual in the US. So, even within English, there’s significant variation from region to region.

P.S.: In the US, bathroom and restroom are synonyms. However, bathroom is more informal, and is more likely to be used in a house than in a public building. Restroom, while more formal, is quite acceptable in all situations and is the best default choice if you’re unsure which is the best option. That said, using bathroom in a formal situation would only be a minor gaffe.

Just indicate that you need to wash your hands. That usually gets the idea across in polite society.

If I am looking for the toilet in an establishment I usually ask for the bathroom or the men’s/women’s room. My understanding is that the term bathroom is perceived as less crude than the word toilet hence why I don’t user the latter.

If I am at a formal event, such as a black tie dinner, I usually just say “excuse me” and then stand up and leave. No need for details in this case.

This question has been asked twice on English SE, the second in a British English context:

• “Toilet”, “lavatory” or “loo” for polite society
• Is there a formal way to say we want to go to the toilet? [closed]

Too summarize that, loo or a bit more polite lavatory would be perfectly fine in the UK, while bathroom can be slightly confusing. People are more direct and also WC and toilet are not deemed inappropriate.

Instead in the U.S. you will find bathroom, washroom or restroom appropriate choices, whereas toilet, lavatory and others are used in slightly different contexts and sound strange if used in your intended context. See this excellent answer here for more details.

This being a basic need of mankind, you will be able to find tons of synonyms, some of them questionable.

If you want to extend your vocabulary, also this blog post might be helpful. Unfortunately it does not distinguish much between British and American English.