The calculation will depend on which Tier you are in, High compliance or Mid compliance.
The Opt-in messages are composed of the following: "Confirmation message", "Welcome message", and "Original message".
Confirmation message. Refer to the "Number of texts needed for this SMS" on each of the Tier tabs (Tier 1: High Compliance and Tier 2: Mid Compliance) to know the exact number of SMS it would consume to send the confirmation message.
Welcome message. This message contains your "Company name" and your "Company's phone number". Usually this message is equivalent to 1 SMS or has a maximum of 160 characters. Note that this might change depending on the length of the company name and the company's phone number.
Original message. This is the message you intend to send your clients. Refer to this article, SMS Concatenation to know more about text credits.
You need to send 100 texts. Each text is 160 characters.
You can have up to 3 text messages (160 x 3) in a single text
In your first text the number of characters used by double opt in requirements are
Tier 1 (High Compliance) = x
Tier 2 (Mid Compliance) = x
Tier 3 (Low Compliance) = 0
If you wish to send 100 texts of 160 characters it would cost you 100 x 160
Plus ONLY IN THE FIRST INSTANCE of sending to a Contact the double opt in text will require x ....(eg 2 texts credits?)
So to send a text with 160 characters it would cost you 100 x 1 + 100 x 2 texts credits = 300 texts credits.
After the contact has opted in each text is calculated at 1 text credit = 160 charachters
eg. a text of 320 characters is two text credits
Log in to your Text Integration account.
Go to the Opt in Manager, then choose whether it's for a High or Mid Compliance.
Type in your message in the field and you will see at the bottom the 'Number of texts needed for this SMS'.
eg. 2 texts
If you have 15 subscribers, then that would be a total of 30 texts.