Marc
20 March 2025 · 2 min read
You've surely noticed: our form limits you to 160 characters per message. Why that number, and not another?
An engineer's story
In 1985, Friedhelm Hillebrand, a German engineer, was looking for a standard size for short messages in the future GSM network. After a few tests on postcards and telexes, he concluded that 160 characters are enough to convey a simple idea.
This limit is written into the GSM 03.40 standard, and it has remained unchanged ever since.
The technical constraint
A standard SMS is encoded in GSM 7-bit, which works out to:
- 7 bits × 160 characters = 1120 bits = 140 bytes
- That's exactly the size of a short message in the GSM signalling channel (not the voice channel).
In other words, the SMS was designed to slip through the network's available "crumbs" without using up voice bandwidth.
What about longer SMS?
When you type more than 160 characters on your phone, your carrier splits them into several concatenated SMS (140 characters each, because of the concatenation headers). Each segment is billed separately.
To keep things free and simple, we therefore limit messages to a single segment of 160 characters. That's more than enough for most uses, and it avoids any nasty surprises on the deliverability side.
