SMS Length Limits and Message Splitting by Encoding – TSG

When sending SMS messages, the length of a message before it is split into multiple parts depends on the type of encoding used. Here's a breakdown of the common encodings and how they affect message length.

1. ISO-8859-1 Encoding (140 Characters)

Single message limit: 140 characters
Multipart message: 134 characters per part

ISO-8859-1 is commonly used for extended Latin characters. When an SMS exceeds 140 characters, it will be split into multiple parts. In each part, 6 bytes are reserved for the UDH (User Data Header), which links the parts together. This leaves 134 characters per part.

2. GSM-7 Encoding (160 Characters)

Single message limit: 160 characters
Multipart message: 153 characters per part

GSM-7 is the most common encoding for standard SMS messages. Messages longer than 160 characters are split into multiple parts. In multipart messages, 7 bytes are reserved for UDH, leaving 153 characters per part.

3. UTF-16 Encoding (70 Characters)

Single message limit: 70 characters
Multipart message: 67 characters per part

UTF-16 is used for messages with special characters, such as emojis. The limit is 70 characters for a single message, but if the message exceeds this, it is split into multiple parts. 6 bytes are reserved for UDH, leaving 67 characters per part.

Why Does the Number of Characters Change in Multipart Messages?
When an SMS is split into multiple parts, each part must contain some extra data to help the recipient's phone reassemble the message. This data is called the User Data Header (UDH), and it typically uses 6 to 7 bytes per message part.

GSM-7 messages reserve 7 bytes for the UDH.
ISO-8859-1 and UTF-16 messages reserve 6 bytes for the UDH.

Because different encodings use a different number of bits or bytes per character, the number of characters available after reserving space for the UDH varies based on how "dense" the encoding is:

GSM-7 uses 7 bits per character.
ISO-8859-1 uses 8 bits (1 byte) per character.
UTF-16 uses 16 bits (2 bytes) per character.

Summary
If your message exceeds the character limit for a single SMS, it will be split into multiple parts:

ISO-8859-1: 140 characters per message, 134 characters per part.
GSM-7: 160 characters per message, 153 characters per part.
UTF-16: 70 characters per message, 67 characters per part.