An individual SMS is limited to 140 octets (140 octets = 140 * 8 bits = 1120 bits) of content data. SMSs can be encoded using a variety of character sets: GSM 7-bit (Default), and 16-bit UTF-16. Depending on which character set is selected, this leads to the maximum individual SMS sizes of 160 for 7-bit (GSM) encoding or 70 for 16-bit (Unicode) encoding, including spaces. When you step over these single message maximum lengths, the maximum character count changes since a small number of characters are now required to be used to associate the individual SMS segments together, so that these segments can then be recombined at the destination as one long message.
The SMS character limits for each encoding type are explained as follows:
7-bit encoded characters (Standard GSM)
UTF-16 (Unicode) encoding