Why is There a 160 Character SMS Limit?

Most mobile phone carriers impose a 160 character limit on short message service (SMS) transmissions as a result of the system through which those transmissions are sent. As such, the limitation is not really a choice for the carrier: 160 characters is often all that the networks can handle at once. Instead of using the primary system for telephone communication, SMS uses a channel designed to send small packets of information between cell phones and cell towers. The 160-character SMS limit is simply a consequence of the originally modest use to which this channel was put. The channel was chosen as the basis for SMS because the system could be implemented easily, with minimal need for new infrastructure.

SMS messages are also commonly referred to as “text messages,” which distinguishes them from messages left by voice. Most modern cellular phones are able to send and receive these messages fairly easily. Brevity is often important to the nature of the message — they’re commonly used to transmit brief notes that are both easy to type and quick to send — but brevity is also normally at a premium because of the character constraints inherent to the medium. In most parts of the world, Latin alphabet text messages are limited to 160 characters each, including spaces.

Some of the newest smartphones are sometimes able to send longer messages that appear to both the sender and the receiver as single, cohesive notes, though in most instances they are actually broken into smaller packets during transmission, then reassembled upon delivery. The technology it takes to do this is constantly evolving, but people with these capabilities may not realize that their messages are actually limited. In many cases the carriers will count the number of messages sent not by the number of times the user presses “send,” though, but by how many 160-character packets have gone out.

A big part of the limitation comes down to basic infrastructure. Cell phones are supported by a network of towers responsible for transmitting calls. These towers constantly communicate with the cell phones they support using a frequency called the control channel. The control channel is used to relay information about the location of phones and towers; it is also used to orchestrate calls. Unlike the calls themselves, these processes require only small exchanges of information. When cell phone providers decided to set up a system for transmitting short messages, this channel was easy to use. All that was necessary was the addition of a Short Message Service Center to receive messages and route them to appropriate towers at an opportune time.

That system was designed by an organization called the Global System for Mobile Communications (GSM). GSM informally analyzed the types of messages that people send to each other and concluded that people would find a message service useful even if it only transmitted between 100 and 200 characters. Restricted by the hardware to a limit of 140 bytes, or 1120 bits, they designed a system that could encode a small number of characters with 7 bits each. This yielded a maximum transmission size of 160 characters.

The limit isn’t necessarily guaranteed, though, and certain factors can actually lower it. When users send longer messages by concatenating a group of SMS packets, for instance, the limit on these individual packets is typically reduced to 153 since each must also contain metadata describing its relationship to the other components of the larger message. SMS users who wish to communicate with other alphabets also are limited to fewer characters. Chinese, Arabic, and Cyrillic SMS typically use UTF-16 encoding, which requires 16 bits per character and thus has an SMS limit of 70 characters per message. Sending concatenated messages using UTF-16 encoding decreases this SMS limit to 67 characters in most cases.

The 160-character SMS limit has become widely known, and many different organizations are designing ways to communicate within this constraint. Several popular social media and microblogging sites are set up with text messages in mind. Some of the most common actually limit transmissions to 140 characters, usually to make room for user names. Many news outlets and politicians also release short messages ready to be transmitted through these services.