Definition · Business Texting
SMS segment
Also known as: SMS segment, message segment, message part
An SMS segment is the billing unit of a text message. A single SMS holds up to 160 GSM-7 characters (or 70 with Unicode, e.g. emoji). Longer messages are split into multiple segments that are billed separately, so a "one" message a buyer writes can count as two or three segments on the invoice.
Carriers and texting platforms bill by segment, not by message. The GSM-7 character set fits 160 characters in one segment; once a message exceeds that, it is sent as a concatenated message and each part carries a few control characters, so the per-segment limit drops to 153. Using any non-GSM character — most emoji, some curly quotes — switches the whole message to UCS-2 (Unicode) encoding, where one segment is just 70 characters (67 when concatenated).
This is the single most common way a texting bill exceeds the estimate. A 170-character reminder is two segments. A 25-character message with one emoji can also be two. A platform's "message credit" or per-message rate is usually a per-segment rate in disguise, so the real cost depends on message length and whether you use Unicode characters.
When comparing per-message pricing across vendors, the like-for-like question is the per-segment rate and whether the platform counts segments transparently — and whether MMS (which is billed at a higher flat rate per message) is charged differently again.