How to Use This Tool
SMS is not measured in characters. It is measured in segments, and segments are what you are billed for. A message that looks fine in your editor can quietly cost three times what you budgeted, because of a single character you cannot see the difference of.
Two alphabets
SMS was designed around GSM-7, a 7-bit alphabet holding the Latin letters, digits, basic punctuation and a handful of extras. Seven bits into the 1,120-bit payload gives you 160 characters in a single segment.
Anything outside that alphabet forces the whole message into UCS-2, a 16-bit encoding. The payload has not changed, so you now get 70 characters. Not the offending character — every character in the message doubles in size.
What triggers it is broader than people expect: any emoji, curly quotes ' and " that word processors insert automatically, the en dash –, accented letters like é and ü, the ellipsis …, and every non-Latin script.
The concatenation cost
Messages over the limit are split into segments and stitched back together on the recipient's phone. That stitching needs a header, and the header lives inside the payload, so each segment drops to 153 characters in GSM-7 or 67 in UCS-2. This is why a 161-character message costs two segments and holds 306 characters' worth of capacity, not 320.
Seven characters that count double
Even within GSM-7, a few characters are stored as two: ^ { } [ ] ~ \ | and the euro sign
€. They live in an escape table, so each one costs two of your 160. A message ending in a curly bracket
can tip over the limit while appearing to be one character short.
How to use this
Paste your message. The counter shows the encoding it will actually be sent in, how many segments that produces, and how much room is left in the current one. If Unicode has been triggered, the exact offending characters are listed at the bottom — usually a curly apostrophe pasted in from a word processor, which is fixed by retyping it as a straight one and instantly more than doubles your limit.
The cost panel multiplies segments by recipients, which is where this stops being trivia. On a 10,000-recipient send, going from one segment to three at $0.0079 is the difference between $79 and $237 for the same message.
