An SMS carries 140 bytes. GSM 7-bit messages can normally contain up to 160 character slots in one SMS; concatenated GSM messages normally have 153 slots per part because the concatenation header consumes space.
If a message contains a character outside the GSM 03.38 default/extension alphabets, this counter switches the entire message to UTF-16/UCS-2 style counting: 70 UTF-16 code units for one part and 67 per concatenated part. Supplementary characters such as many emoji use two UTF-16 code units.