140 bytes is essentially the maximum length of SMS body, which is equal to 160 GSM-encoded characters or 70 unicode-encoded characters.

More illustration:

SMS Maximum length for English and symbol characters is 160 character

SMS Maximum length for Arabic is 70 character

SMS Maximum length for Mixed Arabic and English is 70 character


Based on the following table

Character set

Data_coding parameter

Character set

Max available characters

SMSC default alphabet

0x00

GSM default

160

LATIN-1

0x03

GSM default

140

UCS2

0x08

UCS2

70