SMS Character Counter - Free GSM-7 Checker

The 140-Byte Telecom Payload: Understanding GSM 03.38 (GSM-7) vs. UCS-2 Encoding

Every standard Short Message Service (SMS) frame transmitted across cellular signaling channels is strictly capped at 1,120 bits (140 octets/bytes). Under the 3GPP TS 23.038 specification, when a message uses only characters from the 128-character GSM-7 default and extension alphabets, each character is packed using 7 bits (a septet). Dividing 1,120 bits by 7 bits per septet yields the famous 160-character SMS limit ($1120 / 7 = 160$).

However, the moment a message includes a character outside the GSM-7 character set—such as a Cyrillic, Arabic, Hindi, or Chinese glyph, an emoji, or a typographic curly quote (`U+2019`)—the SMS gateway must re-encode the entire payload using UCS-2 (16 bits / 2 bytes per code unit). Dividing 1,120 bits by 16 bits immediately slashes the single-message capacity to 70 characters.

User Data Headers (UDH) and the Hidden Math of Multi-Part SMS Concatenation

When a message exceeds 160 GSM-7 septets or 70 UCS-2 code units, it cannot be sent as a single PDU frame. Instead, the carrier splits the text into multiple segments and prepends a 6-byte (48-bit) User Data Header (UDH) to every segment containing a reference ID, total part count, and sequence number.

In GSM-7 mode, 48 bits of UDH plus 1 padding fill bit consume 49 bits—exactly 7 septets—reducing each segment's usable text capacity from 160 to 153 septets. In UCS-2 mode, those 48 bits consume 3 full 16-bit code units, reducing each segment's capacity from 70 to 67 characters. Because CPaaS platforms bill per transmitted segment rather than per logical message, an accidental Unicode trigger can multiply enterprise SMS budgets by 200% to 300% overnight.

Eliminating Accidental Unicode Downgrades Before Campaign Launch

Marketing teams routinely draft SMS campaigns in Notion, Google Docs, or Microsoft Word, which silently replace straight apostrophes (`'`) with curly right single quotation marks (`’`) and double hyphens with em-dashes (`—`). Multinook's live septet inspector flags every non-GSM character in bright red and offers instant 1-click transliteration in local browser RAM.