“Bathroom.” Less often, “Ladies’ Room” or “Restroom.”
If I am telling someone what I’m doing, I say “I have to use the bathroom.” If I am in a restaurant, hotel lobby or other public place, I’ll ask to “use the ladies’ room” or “the restroom.”
I used to call it the bathroom, until I used that term for the privy at work, and several co-workers laughed and pointed out that it didn’t have a bathtub.
I should say @zenzen, I call the room where the bath/shower is the bathroom. The room with the toilet in it is the toilet or loo (but in my house, there is a toilet in the bathrooms).
In less enlightened company the bog or more rarely crapper. Crappr is a reference to Thomas Crapper, a plumber who helped to popularise the flush toilet in Victorian England not as many believe its inventor.
I would ask in a restaurant for ‘the ladies’. At home I would say I’m going to the loo. With other people I would say toilet or loo. Most people say toilet in the uk
I should mention that, while on the job with a certain coworker, I would refer to it as the Kye-bo (not sure of the spelling). That is the term Boy Scouts use for an outhouse.
Among friends I used to call it ‘the cludgie’ or ‘the bog’ or just say I was going for a piss, or taking a leak. Nowadays I usually say ‘the toilet’ or ‘the gents’. When in China ‘WC’ came in handy and was widely understood.