The standard SMS message is limited to 160 characters — a constraint that has shaped how text messaging is used for over three decades. Understanding why this limit exists, how it can be extended, and what happens with non-standard characters helps businesses write more effective SMS messages and avoid unexpected delivery issues.
Why 160 Characters?
The 160-character limit dates to the earliest days of SMS development in the 1980s. Researchers studying the maximum length of messages on postcards and in telex messages concluded that 160 characters was sufficient for most short communications. The GSM standard adopted 160 characters as the maximum for a single SMS message, encoded using the GSM 7-bit character set (also called GSM-03.38).
The 7-bit encoding allows 128 characters — sufficient for standard Latin alphabet characters, digits, and common punctuation — to be packed efficiently into the 140 bytes available in an SMS data payload. 140 bytes × 8 bits / 7 bits per character = 160 characters maximum.
GSM 7-Bit vs Unicode (UCS-2)
The standard 160-character limit applies when a message uses only characters from the GSM 7-bit character set — the standard Latin alphabet, digits, and common symbols. However, when a message contains characters outside this set — accented letters (é, ñ, ü), Chinese, Arabic, or emoji — the entire message must be encoded in Unicode (UCS-2), which uses 16 bits per character.
With Unicode encoding, the 140-byte payload holds only 70 characters — less than half the standard limit. This means a message with even one emoji or non-GSM character shrinks from 160 to 70 characters maximum per message segment.
Concatenated SMS (Long SMS)
When a message exceeds the single-segment limit, most modern platforms and devices support concatenated SMS — breaking the message into multiple segments, each carrying a header that tells the recipient's device to reassemble them in order. Concatenated SMS segments are limited to 153 characters each (7 bytes are used for the concatenation header), so a two-segment message holds 306 characters total, three segments hold 459, and so on.
Each segment is billed as a separate SMS by carriers, so longer messages cost proportionally more to send when charged per message.
Practical Implications for Business SMS
Keep messages under 160 characters when using standard Latin characters to ensure single-segment delivery and lowest cost. Avoid emoji and special characters in high-volume business alerting to prevent inadvertent Unicode encoding that halves your character budget. Test message length in your SMS platform before launching campaigns — what appears short may contain invisible special characters that trigger concatenation.
