SMS segment: definition and how it works
Published and updated
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.
How it works
The mobile network never sends a message; it sends segments. A segment carries at most 140 octets of user data, and everything about SMS cost and length follows from that number and the encoding chosen for the text.
In the GSM-7 alphabet a segment holds 160 characters. In UCS-2, used whenever a character falls outside GSM-7, it holds 70. When the text needs more than one segment, the sender adds a six-octet concatenation header to every part, and the capacity per segment drops to 153 GSM-7 or 67 UCS-2 characters. The rules for both encodings and for concatenation live in the 3GPP specifications for SMS alphabets and for the technical realisation of the service.
The number of segments is the number the bill shows. A cloud provider's per-message price is really a per-segment price, so a 200-character notification costs twice a 150-character one. A SIM plan with a message allowance is drawn down per segment too.
Segment counting also affects delivery. Each segment is a separate transmission with its own chance of delay, and the phone or gateway that sends them does so one after another. Short messages are cheaper, faster and more reliable, which is why templates for OTP and alerts are written to fit in one segment.
- 3GPP TS 23.040, Technical realization of the Short Message Service (SMS), defines the short message user data and its concatenation. portal.3gpp.org, checked on September 5, 2026
- 3GPP TS 23.038, Alphabets and language-specific information, defines the 7 bit and UCS2 encodings that set the per segment character counts. portal.3gpp.org, checked on September 5, 2026
How this applies with textbee
The textbee plans include a number of messages a month and each segment counts as one. Check a template with the segment calculator before you ship it; a code, an expiry and a sender name fit easily in 160 GSM-7 characters.
Related terms
- GSM-7: 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.
- 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.
- 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.
Frequently asked questions
Why did my 160 character message cost two segments?
One character outside the GSM-7 alphabet, often a curly quote, an emoji or an accented letter, switched the message to UCS-2, where a single segment holds 70 characters. Replace the character or shorten the text.
Sources
- SMS segment, portal.3gpp.org, checked on September 5, 2026
- SMS segment, portal.3gpp.org, checked on September 5, 2026