SMS encoding and message length

Two main encodings

A single SMS carries up to 140 bytes of user data. How many characters fit depends on the encoding, which is defined in 3GPP specification TS 23.038.

If any character in a message is not in the GSM-7 alphabet, the whole message is usually sent as UCS-2. One curly quotation mark or emoji in an otherwise plain English message can cut the limit from 160 to 70 characters.

Extension characters count twice

The GSM-7 alphabet has an extension table reached with an escape character. Each of these characters uses two positions: ^ { } \ [ ~ ] | € and the form feed. A message of 160 characters that includes one euro sign no longer fits in one SMS.

The basic GSM-7 alphabet includes the Latin letters, digits, common punctuation, a few accented letters such as é, è, ü and ñ, and some Greek capital letters. It does not include many common characters, such as curly quotes, the en dash or most accented capitals. Some countries can use national language shift tables defined in TS 23.038; support for them depends on the networks and provider.

Long messages and segments

Longer text is split into segments that the handset joins back together. Each segment carries a small header (the user data header) identifying the part, which is why the per-segment limit is lower: 153 GSM-7 characters or 67 UCS-2 characters. Providers usually charge per segment. An extension character is never split across two segments.

Over SMPP, long messages are sent either as several submit_sm requests with a user data header, as one request using the message_payload field, or with the SAR fields, depending on what the provider supports. The data_coding value states the encoding, for example 0 for the default alphabet and 8 for UCS-2.

Tips to control length

• Replace curly quotes, long dashes and ellipsis characters with plain ', ", - and ... when the language allows.
• Keep one-time passcode messages short and free of emoji.
• Watch for characters pasted from word processors, which often add curly quotes and invisible characters.
• Test the exact text, including personalised values, before a campaign.

Use the SMS length calculator to check a message. It runs in your browser and estimates the encoding and segments; the final count is decided by the provider and networks that carry the message.

Reference

• 3GPP TS 23.038: alphabets and language-specific information.
• 3GPP TS 23.040: technical realisation of SMS, including concatenation.