Are you (singular formal) = ¿Es usted?
Eres can also be used for both singular informal and formal if you leave off tú or usted
Alternatively, Ser can be used instead of Eres, and Está can be used instead of Es
Eres = Estás
Es = Está
Examples:
¿Eres estudiante de español?
Are you a Spanish student?
¿Es usted abogado?
Are you a lawyer?
¿Estás contento?
Are you happy?
¿Está lloviendo?
Is it raining?