Two alphabets
An SMS is written in one of two encodings. GSM-7 covers plain Latin letters, digits and common signs such as @ and $. Anything outside it, for example emoji, curly quotes or Arabic, Cyrillic, Thai or Chinese script, switches the whole message to Unicode (UCS-2).
How many characters fit
One GSM-7 part holds 160 characters. One Unicode part holds 70. When a message is longer, it is split and each part loses a few characters to the header that joins them again: 153 per part for GSM-7 and 67 for Unicode.
Characters that count twice
Some GSM-7 signs take two places: ^ { } \ [ ] ~ | and the euro sign. A message of 158 letters with one euro sign is still one part; with three it becomes two.
Why one emoji matters
A 150 character message in plain text is one part. Add one emoji and the message becomes Unicode, so the same text now needs 3 parts and costs three times as much.
Keeping messages short
Use straight quotes and a plain hyphen, drop emoji from bulk sends, and check the part counter before sending. The calculator on our home page and the Send page show the encoding and the parts as you type.
