SMS messages are just 140 bytes long. This means that they can carry 160 GSM-7 characters or 70 UCS2 characters (more on encoding here). Messages longer than this are supported by splitting the message into parts, sending them individually, then reassembling them on the receiving phone. These are called multipart or long SMS message. The splitting and reassembling happens automatically. If you send a long message from your phone you won’t know that your phone has split the message for transmission and the receiving phone will display it as a single long SMS.
This is important for SMS marketers because each part of a long SMS message costs the same as a single short message. This is a feature of the SMS protocol and applies to all SMS messaging regardless of provider.
The parts of a multipart message have a small portion of their data reserved for a header which stores the total number of parts and the position of this particular part (e.g part 2 of 3). This information is used by the receiving phone to correctly reassemble and display the message. In the case of GSM-7 encoded messages the header takes 7 characters worth of data meaning the parts each carry 153 characters, for UCS2 message the parts carry 67 characters.
| Encoding | Single SMS | Long SMS part |
| GSM-7 | 160 | 153 |
| UCS2 | 70 | 67 |