SMS's can be segmented if they are too long or use non-standard characters, messages are charged on a per segment basis and not per message. Customers can be frequently charged for many segments if they use non-standard characters.

For GSM-7:  1 Message to 1 x recipient: 1 - 160 characters

For joined segmented Messages to 1 x recipient:

• 1 – 153 characters = 1 Message
• 154 – 306 characters = 2 Messages
• 307 – 459 characters = 3 Messages
• 460 – 612 characters = 4 Messages
• 613 – 765 characters = 5 Messages
• 766 – 918 characters = 6 Messages - (recommended max character length for a joined message)

For Non-GSM (Unicode):    1 Message to 1 x recipient: 1 - 70 characters

For joined segmented Unicode Messages to 1 x recipient:

• 1 – 67 characters = 1 Message
• 68 – 134 characters = 2 Messages
• 135 – 201 characters = 3 Messages
• 202 – 268 characters = 4 Messages
• 269 – 335 characters = 5 Messages
• 336 – 402 characters = 6 Messages - (recommended max character length for a joined messages)

What is a GSM-7 Character and what is a Unicode Character?

The base character set for GSM-7 can be found at the link below:

https://en.wikipedia.org/wiki/GSM_03.38#GSM_7-bit_default_alphabet_and_extension_table_of_3GPP_TS_23.038_.2F_GSM_03.38

To go further on why we segment like this, below is a handy excerpt from the above link:

"In a standard GSM text message, all characters are encoded using 7-bit code units, packed together to fill all bits of octets. So, for example, the 140-octet envelope of an SMS,with no other language indicator but only the standard class prefix, can transport up to (140*8)/7=160, that is 160 GSM 7-bit characters (but note that the ESC code counts for one of them, if characters in the high part of the table are used)."