The uncomplicated guide to concatenated SMS

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Concatenated SMS

You know each SMS has a limited number of characters. That limit differs depending on the language and character set of the phone in use, so the trick is to be as concise as possible. For those messages you can't make any shorter, however, the solution is concatenated SMS which splits messages into smaller parts to ensure the entire SMS is received in a single text. This article explains how concatenated SMS works in multiple messaging languages.

Count your ABCs

Typically an SMS message has a maximum payload size of 1120 bits (140 octets or bytes). Phones that support Latin-based alphabet languages like English, French and Spanish use GSM character encoding, which takes 7 bits per character, thus allowing a maximum of 160 characters per SMS. On the other hand, phones that support languages not using a Latin-based alphabet, like Arabic or Chinese, use a Unicode character encoding, usually Unicode Transformation Format (UTF-8). Unicode encoding takes 16 bits per character, allowing a maximum of 70 characters per text message. As you can see, there is a big difference how many characters it may take to convey your message, depending on whether your recipient's phone uses GSM or UTF-16.

Concatenated SMS ensures that your entire message will be delivered, whether it takes one text message or three message parts - the end user will always receive your SMS as a single message.

Here's how concatenated SMS works

Concatenated SMS splits up your message into smaller parts, which are sent as individual SMS messages and recombined on the recipient's handset. This concatenation process requires additional information to be added to each concatenated text message for identifying and ordering each message part. This information is known as the user data header (UDH) and takes up 48 bits of each SMS payload, which means only 1072 bits are left over for the content of the message.

This table illustrates the total number of characters available for your concatenated SMS message content, depending on the alphabet and character set used:


 Number of SMS

 Maximum GSM characters

 Maximum Unicode characters

 1 standard SMS



 2 concatenated SMS



 3 concatenated SMS



 4 concatenated SMS




SMS concatenation with Clickatell

To ensure that your messages are delivered, Clickatell supports concatenated SMS with a default setting of 3 message parts when you sign up for a Clickatell Platform account. As with every mobile provider, each message part is charged as 1 SMS and is subject to our available coverage and pricing. You can easily change the default number of message parts allowed either during your integration setup, under the 'Settings' tab. To ensure your messages are being delivered as you intended, sign up for a free Clickatell Platform account today and take advantage of unlimited free test messages in our sandbox environment.