As far as character set is concerned, there are 2 types of SMS.
1) ASCII (7 bits) SMS
2) Unicode SMS – If 1 or more char is a non-ASCII, then it is a Unicode SMS. Read what is Unicode.
SMS length for ASCII SMS
Single SMS: 160
Multiple SMS: 153n , where n is number of SMS and n>1
1st message: 160 characters
2nd message: 160 + 146 characters
3rd message: 160 + 146 + 153 characters
Pitfalls: This is important and it applies to ASCII SMS only. Some characters are regarded as 2 characters, as they actually need more than 7 bits to represent. ASCII SMS table shows the Hex (leftmost column) used to represent each character. You will find that there are 10 characters (form feed, ^, [, ], …) requires 2 Hex to represent. Hence, these 10 characters need special attention.
SMS length for Unicode SMS
Single SMS: 70
Multiple SMS: 67n
1st message: 70
2nd message: 70 + 64
3rd message: 70 + 64 + 67