GSM-7: definition and how it works
Published and updated
GSM-7 is the default 7-bit character set for SMS, defined in 3GPP TS 23.038, which fits 160 characters into the 140 bytes of a single text message.
How it works
An SMS carries 140 octets of user data. If every character took a byte that would be 140 characters, but the original designers squeezed more in by using seven bits per character. Seven bits give 128 code points, enough for the Latin letters, digits, common punctuation and a handful of accented and Greek characters. Packed into 140 octets, that is 160 characters, which is where the familiar limit comes from.
The alphabet is defined in 3GPP TS 23.038, Alphabets and language-specific information, alongside the rules for other encodings. It also defines an extension table reached through an escape character. Characters in the extension table, including the square brackets, curly braces, backslash, caret, pipe, tilde and the euro sign, take two of the seven-bit slots each. A message full of them fills up faster than its visible length suggests.
Anything outside the basic and extension tables cannot be expressed in GSM-7 at all. One emoji, one curly quote pasted from a word processor, or one character from a script such as Arabic or Devanagari forces the whole message into UCS-2, which drops the single-segment limit from 160 characters to 70.
That is why message encoding matters for cost. Two messages of the same visible length can be one segment and three segments.
- 3GPP TS 23.038, Alphabets and language-specific information, defines the GSM 7 bit default alphabet and its extension table. portal.3gpp.org, checked on September 5, 2026
How this applies with textbee
textbee sends whatever text you pass, and the phone applies the encoding rules. Keep templates to GSM-7 characters when you can, and check them with the segment calculator, because each extra segment is another SMS against the SIM plan.
Related terms
- UCS-2: UCS-2 is the 16-bit character encoding SMS falls back to when a message contains any character outside the GSM-7 alphabet, which limits one segment to 70 characters.
- SMS segment: An SMS segment is one physical text message of up to 140 bytes, the unit that networks carry and providers bill, so a long or non-Latin message counts as several segments.
- Concatenated SMS: A concatenated SMS is a message longer than one segment that the sender splits into several SMS carrying a shared header, so the receiving phone can reassemble them into one text.
Sources
- GSM-7, portal.3gpp.org, checked on September 5, 2026