The History Behind SMS Message Lengths
SMS is a standardized communication protocol that enables devices to exchange short text messages and was defined as part of the 1985 GSM protocol. Originally it was designed to "fit in between" existing signaling protocols which is why SMS length is limited to 160 seven-bit characters. It was subsequently codified into the SMPP signalling protocol that transmits SMS and is limited to precisely 140 bytes (or 1120 bits).
Where this gets tricky is because GSM-7, the original character set designed for SMS only has the ability to denote 128 different characters in those 7 bits. So if you want to include more Latin or eastern characters we have to upgrade to UCS-2 characters (think characters like á and ú) - and with that format, we can only send 70 characters per segment, meaning a 71-character message would require two segments.
Twilio has a great article detailing this history here:
Estimating the number of segments needed to send your message
When creating an outbound message, we'll provide you with an estimate of your sending cost, along with the number of segments required to send each message:
Keep in mind if you're electing to send a survey with your message, the survey question and each option's text is included in your message size calculation. For this reason, for outbound questions, we recommend starting with a smaller question with 2-4 options to reduce your overall cost.
We automatically convert as many characters as possible from the advanced UCS-2 to GSM7 format for you in an effort to preserve 140-character segment sizes. When that's not possible, we'll upgrade to UCS-2 and factor in the 70 character limit for your message segments.
See how your segment totals are calculated
If you'd like a look under the hood on calculating total segments needed, we recommend this segment size calculator: