An SMS holds 160 characters because one message has to fit in 140 bytes, and the standard SMS alphabet packs each character into 7 bits. 140 bytes is 1,120 bits, and 1,120 divided by 7 is 160.
So 160 is not a round number somebody liked. It's what is left when you divide a fixed amount of space by the size of a letter.
Why only 140 bytes
SMS was designed to travel over the signalling channel that GSM networks already used to set up calls, in the gaps when that channel was idle. That channel carried small packets and no more, so a text had to be tiny to fit. 140 bytes was what the packet allowed.
Where 160 was chosen
In the mid-1980s Friedhelm Hillebrand chaired the committee that wrote the non-voice services part of the GSM standard. By the usual account he looked at how long postcards and telex messages typically were, and decided that about 160 characters was enough to say something useful.
That the answer and the packet size matched so neatly (a 7-bit alphabet in 140 bytes gives exactly 160) is part of why the number stuck. The first text message anyone sent, on 3 December 1992, was "Merry Christmas", from the engineer Neil Papworth to a Vodafone director in the UK. It used 15 of the 160.
Why one emoji drops it to 70
The 7-bit alphabet only has room for about 128 characters: English letters, digits, common punctuation and a few accented letters. Anything outside it, such as an emoji, a Hindi letter or a curly apostrophe, needs 16 bits per character instead. 1,120 divided by 16 is 70.
And the whole message switches, not just the one character. One curly apostrophe in an otherwise plain 100-character text is enough to turn it from one message into two. The SMS character counter shows which character did it.
What happens past the limit
A longer text is cut into parts that the receiving phone stitches back together. Each part spends 6 bytes on a header saying where it belongs, which is why a joined message holds 153 characters per part, or 67 for Unicode, rather than 160 and 70.
The parts arrive as one message in your inbox. But businesses sending SMS through an operator are usually billed per part, which is why a 161-character reminder costs twice a 160-character one.
The tweet that is 140 for the same reason
Twitter was first built to work over SMS. The story is that its limit came from the 160 in a text, minus 20 characters reserved for the username. That left 140, the number tweets kept until 2017.
Does it still matter?
Not on WhatsApp, iMessage or most modern chat apps, which have no 160-character limit and send emoji freely. It matters wherever a message is a real SMS: bank alerts, delivery updates, two-factor codes, a text to a phone without data, and anything a business sends in bulk.
It also explains something people notice and never trace. Paste a fancy styled name into a text and the limit collapses, because every styled letter is outside the 7-bit alphabet. That's the same arithmetic, quietly doing its work.
Where to go from here
- SMS character counter paste a message to see its parts and which character is costing you space
- Font generator and why a styled name eats the limit