What is an SMS segment?
A segment is the unit SMS is actually billed in: 160 GSM-7 or 70 UCS-2 characters. Longer texts split into parts of 153/67 characters — and every part is charged as a full message.
Definition
An SMS segment is the atomic unit of SMS transmission and billing. One segment carries 140 bytes: 160 characters in GSM-7 encoding or 70 in UCS-2. Text longer than that is split into a concatenated (multipart) message: each part carries a 6-byte User Data Header (UDH) telling the handset how to reassemble the whole, which shrinks per-part capacity to 153 GSM-7 or 67 UCS-2 characters. The handset shows one seamless message; the network — and your invoice — sees N separate messages.
Segment math is where SMS budgets quietly leak. A 161-character GSM-7 message is two segments (2× cost for one character); a 154-character message with a single emoji is three UCS-2 segments. Carriers may also deliver parts out of order or drop one, so very long messages fail more. Practical rules: keep transactional templates under 160 GSM-7 characters, watch for characters that flip the encoding, and remember that URL shorteners and personalisation tokens change segment counts per recipient.
Why it matters
Providers price per segment, not per message — so segment count is the single biggest controllable driver of SMS spend. Auditing your top ten templates for encoding flips and off-by-a-few overflows routinely cuts a messaging bill by double-digit percent.
See it in action
Count segments and cost before sending →