About GSM-7

GSM, or Global System for Mobile Communications, defines how characters are counted in an SMS messages.

It is important to understand this GSM specification in order to write your messages effectively and effitiently and to minimize message concatenation into multiple parts since all SMS carriers and providers charge by the SMS part.

A single SMS message that follows the standard can contain up to 160 characters. To ensure compliance with the GSM 3.38 protocol, all the characters used in the message must belong to the 7-bit default alphabet, which encompasses ASCII characters as well as some accented characters such as u umlaut (ü) and e with grave (è).

If any character outside of the aforementioned set is used in the message, it will be considered a Unicode SMS. Due to its distinct character encoding, a Unicode SMS has a character limit of 70 characters. Additionally, in the event that the message contains more than 160 characters (or 70 characters in the case of Unicode SMS), it will be divided into multiple parts, and the corresponding charges will be applied.

GSM-7 Encoding details

GSM-7 is a widely used character encoding standard that allows for efficient transmission of commonly used letters and symbols in various languages on GSM networks by packing them into 7 bits each. SMS messages are sent in 140 8-bit octets, which means that a GSM-7 encoded SMS message can contain up to 160 characters.

The GSM-7 character encoding standard is specified in the GSM 03.38 standard and is universally supported on GSM networks. In languages where the number of commonly used symbols exceeds 128, the use of GSM-7 encoding is mandatory. However, in such cases, local language support is achieved by using shift tables or switching to (16-bit) UCS-2 encoding.