Mass Texting is charged per segment, and each segment is 160 characters. When you are creating a Mass Text, the font will turn red once you exceed 160 characters. There’s also information on this screen that explains what the cost is per “segment”. Although a segment is 160 characters, when you use emojis or foreign symbols, it can exceed the character limit and result in multiple segments being charged.
You can use this tool to determine how many segments a text is. We also recommend using a link shortener to limit the number of characters.