The number of units used for a message depends on its length and the encoding it requires. A message of 160 characters long (including spaces) is what carriers consider as a segment and what we count as a unit, assuming this message doesn't contain a special character not in the GSM-7 encoding set.
GSM-7 is the standard alphabet for SMS messages, written up in the standard GSM 03.38. It is always supported on GSM networks. In languages with more than 128 commonly used symbols, GSM-7 is mandated but local language support is implemented with shift tables or by changing text encoding to (16-bit) UCS-2 encoding. The basic character set for GSM-7 can be found here.
If you include any non GSM-7 characters in your message body, we will automatically fall back to UCS-2 encoding (which will limit units to 70 characters each). Additionally, Longer texts prepend a User Data Header of 6 Bytes (this instructs the receiving device on how to assemble messages), leaving 153 GSM-7 characters or 67 UCS-2 characters for your message per unit.
GSM-7
1 - 160 characters = 1 unit
161 - 306 characters = 2 units
307 - 459 characters = 3 units
UCS-2
1 - 70 characters = 1 unit
71 - 134 characters = 2 units
135 - 201 characters = 3 units
To help you manage your costs and the number of units used for a message we conveniently added a character/unit counter on every message composing page.
Note: For international texting (out of USA) the number of units a message requires is different depending on the Country and carrier being texted. Visit the International Pricing page to see how many international units a text requires per unit.