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How is the actual length of SMS calculated

GSM 03.38 character set defines GSM 7 bit alphabet (with extension table) and GSM 8 bit data encoding.

GSM 7 bit alphabet

Basic character set:
@£$¥èéùìòÇ'Øø'ÅåΔ_ΦΓΛΩΠΨΣΘΞ'ÆæßÉ !"#¤%&'()*=,-./01234
56789:;
<=>?¡ABCDEFGHIJKLMNOPQRSTUVWXYZÄÖÑÜ'¿abcdefghi
jklmnopqrstuvwxyzäöñüà

Extension table characters:
|^€{}[]~

When you say that an SMS is 160 characters long, this means, that it can contain 160 characters from the Basic character set above.

Every character from the extension table (|^€{}[]~) takes up two regular characters!

Example: When you type text “Hello World! ^_^” it is 16 characters long, but actually takes up 18 characters on a regular message (due to two carets “^”)!

GSM 8 bit data encoding

When your message contains even one character that is not included in the GSM 7 bit alphabet (for example š, ž, –, ´, “), then the message is sent in 8 bit data encoding.

The length of one message is 70 characters for 8 bit SMS instead of 160 characters for 7 bit SMS!

Multipart message

7bit: 160 + 160 = 320 306

8bit: 70 + 70 = 140 134

When you send a message, that exeeds the length of one message, the message is sent as multipart message. Combining two messages as one large message takes up a few extra characters, so the length of two regular messages is 306 characters instead of 320 characters. This also implies to SMS sent with GSM 8bit data encoding.

Live SMS length calculator

Messente also has a free web-based SMS length calculator - give it a try on http://messente.com/documentation/sms-length-calculator

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