The standard SMS can contain up to 160 characters. However, these characters must be part of the so-called GSM-7 bit default alphabet. This alphabet contains all ASCII characters, some accented characters and some Greek symbols. On this page you can see a complete overview of the GSM-7 bit default alphabet:
In general, this means that most text messages in standard English can can contain up to 160 characters. However, if you need to characters from the Cyrillic alphabet, or need to type the á, which is not in the GSM-7 bit default alphabet, for example, the SMS will need to be a Unicode SMS and can then contain only 70 characters.