Single text messages are limited to 160 characters, including spaces. The maximum length of two-part or three-part multipart text messages is 306 (2 x 153) and 459 (3 x 153) characters, respectively. 

One can send up to 1000 characters as multipart message which will be split into 7 messages internally. This refers only to messages that contain the standard GSM 03.38-character set. 

Each multipart text message is limited to 153 characters rather than 160 due to the need for user-data headers (UDHs) information. 

Mobile phones use UDH information to enable them to link long messages together so that they appear as single SMS messages in recipient’s phone inbox.

Number of SMS
Number of characters in the linked SMS
1
160 characters
2
306 characters (2 x 153)
3
459 characters (3 x 153)
...
..
7
upto 1000 characters (6 x 153) + (1 x 82)