160, 153, 70 and 67
An SMS carries 140 bytes. In the GSM-7 alphabet that is 160 characters; in UCS-2, which Cyrillic, Chinese, Arabic and most emoji need, it is 70. A longer message is split into parts that the phone joins back together, and every part gives up some room to the joining header: 153 characters per part in GSM-7, 67 in UCS-2.
One character can double the cost
A message is sent in one encoding. A single character outside GSM-7, such as a curly quote, an emoji or a letter like ï, switches the whole message to UCS-2 and cuts its room from 160 to 70 per SMS. The counter lists those characters so you can replace them.
The GSM-7 extension characters ^ { } \ [ ] ~ | € stay in GSM-7 but count as two.
What the counter assumes
It follows 3GPP TS 23.038: the GSM 7-bit default alphabet and its extension table. Some networks use national language tables (Turkish, Spanish, Portuguese, Indian languages) that fit more letters into GSM-7; whether yours does cannot be known from the text, so the counter does not assume it. The text never leaves your browser.
Receive SMS on a number of your own
A private number from SMS Activate App receives SMS and calls in your dashboard, and it is yours alone while you keep it.