Let’s start with the non-technical reasoning and keep it simple: 1 SMS message is limited to 160 characters. Thus, if you type a message that is 175 characters in length, it is considered 2 messages (or credits).
Now for the more technical explanation: This is an industry standard that Sire and all SMS companies are bound to. SMS was created a few decades ago as a standardized communication protocol that was designed to exchange short messages between devices. It was inherently designed with a limit of 1120 bits. English characters are 7-bits each. 1120/7=160. This is why one SMS message is defined as 160 characters.
To make matters more complicated, there’s the issue of “concatenated SMS”. That sounds fancy but I’ll try to keep it simple. Let’s say you’re sending a message longer than 160 characters. That text will be split into more than one message when sent. Each message segment is assigned an ID which notes the order and how many segments are being sent. This way, the message can be reassembled correctly for the recipient. Imagine the ID as page numbers. If you give someone on a 5 page document, you want each page numbered so the recipient will know the order of the pages and if any of them are missing. In the text marketing world, the first page is your standard message and pages 2-5 would be concatenated messages. This ID requires a few bytes of data which will eat into your character limit. For standard messages, the first “page” of your message has a limit of 160 characters and each additional “concatenated” message has a limit of 153. If you’re using Unicode text, the first message limit is 70 and each concatenated message is 63. (learn more about Unicode text here)
It’s important to note that this actually works in your favor for marketing. Consumers appreciate the direct nature of text messages. They are growing sick of “marketing fluff.” Keeping your messages short & sweet will not only keep your credit usage low but also appeal to more of your customers.