General Question

sferik's avatar

Why does using emoji reduce my SMS character limit to 70?

Asked by sferik (6074 points ) September 12th, 2011

When I start typing a message in the Messages app on my iPhone, it shows the character limit at 160, but as soon as I type a single emoji character, the limit drops to 70.

It makes sense that using double-byte characters would cut the maximum number of character in half (80), by why is it less than half (70)?

I assume there’s some extra header being sent that indicates the message is encoded differently. I’m curious what the actual contents of that header is and why it takes up 20 characters (or 10 double-byte characters).

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7 Answers

XOIIO's avatar

Its more data than a letter.

sferik's avatar

@XOIIO How much more? Twice as much? Then shouldn’t the character limit be 80 instead of 70?

XOIIO's avatar

@sferik Beats me, its whatever rules they got set up.

jrpowell's avatar

This might help.

sferik's avatar

@johnpowell Thanks for going to the trouble of doing that, however, just because emoji is base64-encoded over email doesn’t mean it’s sent the same way over SMS.

I think I found the answer in RFC 5724: “SMS messages have a maximum length of 160 characters (7-bit characters from the GSM character set [SMS-CHAR]), or 140 octets.”

Assuming emoji characters occupy 2 octets, the 70-character limit makes perfect sense.

jrpowell's avatar

Good find. That makes sense.

lifeflame's avatar

Incidentally, the same thing happens to me whenever I type a Chinese character on my phone. Must be a similar coding thing.

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