How to ask - I need to potty.
How you say it politely when you are among a group of people?
¡Perdóneme! Tengo que hacer pis o pipi o orinal.
¡Perdóneme! Necesito hacer pis.
Please answer before it is too late! 
11 Answers
Polite ways to excuse yourself:
¡Perdonen,
a) debo ir al cuarto de baño/water/baño/servicio.
b) ¿me pueden disculpar un momento?
c) ¿me disculpan?
Zoltán asked:
How you say it politely when you are among a group of people?
I answered:
Tal vez: "Perdóneme. ¿Dónde está el baño (o 'el aseo' o 'el WC', o ...)?
Zoltán responded:
If I would not know where the WC is, I would ask "¿Dóde está el cuarto de caballeros?" or something like that. But that was not my question.
You made me laugh. Thanks! I needed that. 
I'm sorry, but please indulge me for a moment. I simply must explain what was going through my mind when I saw this question pop up the other day.
I was among the first to read this question (it showed '0 vistas') and my first thought was that it was a trick question. After all, Zoltán seems to have been around the forum for a while. But then I thought:
"I haven't really been here long enough to know Zoltán that well. Maybe he's not joking. And then I thought ‘What if I was in his shoes?’ What if I was at a restaurant with some friends and really did have to go potty, and bad. What if I was hemmed in by the table and chairs and couldn’t just get up and walk out. And what if I had already been racking my brain for a while trying to think of how to say it politely but was drawing a blank. That would mean the situation would be getting critical!”
“But... what if I had an iPhone with the new SpanishDict App? Of course! I’d whip it out, type out my S.O.S. message and send it on its way to the internet asking for help from my SpanishDict friends. And just to make sure everyone realized this was an urgent matter I would certainly have added “Please answer before it’s too late!”
“And then,” I continued thinking to myself, “What type of answer would I be hoping for? There might be a lot on the line... my employment, my friendships, my self-respect... I definitely would not want an answer that might make me look foolish…"
So, there I was, with all of this was racing through my head as I typed out the best answer I could think of, but quickly! All I can say is I’m glad more people jumped in and saved him from a potentially embarrassing situation. 
But now, Zoltán, you’ve got my curiosity up. Please tell us, just what were the circumstances that prompted this question of yours? Also, I seem to recall you mentioning in another thread that Hungarian is your mother tongue. So, how would you politely say “I need to potty” in Hungarian? And how would one translate that literally into Spanish or English?
Throughout the SW US and northern Mexico, the room is often called el servício.
I should have know "Zoltán" was serious.
Ok here is my shot at this -- you are correct but you need the verb for "go" -- I know you know that.
- "¡Perdóneme! Tengo que ir a hacer pipi." This is how I would say it -- but probably never would. ja ja ja.
Discúlpame/Discúlpeme/Disculpadme/Discúlpenme. Vuelvo enseguida.
¡Perdóneme! Necesito hacer pis.
Wheras this form is correct and chidren use the words like this I find this very unbecoming with adults.
You should ask for the bathroom. Would you say in a group with people you don't know too well:
I have to pee, I have to take a piss...sounds rude to me.
Among the dramatic..."I must go shake the dew off my lily"
In Britain they have a lovely expression: I am going to shed a tear for Nelson
Dunno if this helps, but I know in class we used to say, "¿Puedo ir al baño por favor?" I mean plain and simple, that means, "Can I go to the restroom please?" lol
If I would not know where the WC is, I would ask "¿Dóde está el cuarto de caballeros?" or something like that. But that was not my question.
Tal vez: "Perdóneme. ¿Dónde está el baño (o 'el aseo' o 'el WC', o ...)?
I'm not sure what country you are in. 
