Issue

How much characters can I fit in an SMS message? What is the effect of character set such a Unicode or GSM-7 on the message length?

Product

• Messaging

Resolution

Multi-part messages are the result of the message length of an SMS message. A generic SMS can contain 160 characters, as long as you stick to the GSM-7 character set. When you exceed the 160 characters the message will be send as a multi-part (concatenated) message, and every part can contain up to 153 characters.

In the GSM-7 character set, there are a few extended characters | ^ € { } [ ] ~ \ that will use two 7-bit byte instead of one. So a message that contains one € sign, can only be 159 characters long.

If you use other characters, such a Cyrillic or Chinese characters, emoji's or specific accented Latin characters, the message will be sent as a Unicode message (UCS-2). Within the Unicode character set a single message can contain up to 70 characters, if you exceed the 70 characters a single message can only contain 67 characters. Where emoji's will consume 2 characters each.

The Campaigns editor will indicate the amount of messages a certain text will consume while composing.

Cause

From Wikipedia:

Transmission of short messages between the SMSC and the handset is done whenever using the (MAP) of the protocol. Messages are sent with the MAP MO- and MT-ForwardSM operations, whose payload length is limited by the constraints of the signaling protocol to precisely 140 bytes (140 bytes * 8 bits / byte = 1120 bits).

Short messages can be encoded using a variety of alphabets: the default GSM 7-bit alphabet, the 8-bit data alphabet, and the 16-bit UCS-2 alphabet. Depending on which alphabet the subscriber has configured in the handset, this leads to the maximum individual short message sizes of 160 7-bit characters, 140 8-bit characters, or 70 16-bit characters. GSM 7-bit alphabet support is mandatory for GSM handsets and network elements, but characters in languages such as Hindi, Arabic, Chinese, Korean, Japanese, or Cyrillic alphabet languages (e.g., Russian, Ukrainian, Serbian, Bulgarian, etc.) must be encoded using the 16-bit UCS-2.

Larger content (concatenated SMS, multipart or segmented SMS, or "long SMS") can be sent using multiple messages, in which case each message will start with a User Data Header (UDH) containing segmentation information. Since UDH is part of the payload, the number of available characters per segment is lower: 153 for 7-bit encoding, 134 for 8-bit encoding and 67 for 16-bit encoding. The receiving handset is then responsible for reassembling the message and presenting it to the user as one long message. While the standard theoretically permits up to 255 segments,​ 10 segments is the practical maximum with some carriers.