What are Bandwidth's SMS character limits and concatenation practices?


Mara Wallace


What is the character limit for SMS text messages?

The maximum number of characters per single message being sent to carriers depends on the encoding used, while the encoding used depends on the content of the message. The HTTP API limits SMS messages to a maximum length of 2048 characters.

See this support article for types of encoding. For example:

  • A message containing text characters only will be encoded using GSM-7
  • A message containing emojis will be encoded using UCS-2

Here's the maximum number of characters that can be sent to carriers in a single SMS segment:

Message Type Characters used in the message Encoding Max characters/ message(without UDH)
Hello - good morning Text GSM Standard GSM-7 160
 image2.png Emoji Unicode UCS-2 70
 image1.png Unicode Unicode UCS-2 70

How does Bandwidth send concatenated (long) messages?

Depending on the message content (plain text, emojis, special characters, etc.), Bandwidth will use either the GSM-7 or UCS-2 encoding to send it. Each encoding has limitations to the number of characters that can be sent:

  • 160 characters for GSM-7 (e.g. Latin-1/9 and GSM8).
  • 70 characters for UCS-2 (e.g. message with emojis)

When you send a message whose text is longer than the maximum number of characters per segment, Bandwidth will automatically split the message for you, add a special header (User Data Header), and send multiple SMS to carriers.

What is a User Data Header?

The User Data Header (UDH) takes up 6 bytes and instructs the receiving device how to reassemble the segments so that your whole message will be shown as one SMS on the receiving handset. The maximum number of characters per concatenated (long) message is slightly reduced due to the inclusion of concatenation headers (UDH).

Can you explain the math around characters and segments?

  • SMS messages are sent in 140 bytes
  • 1 byte = 8 bits
  • In GSM encoding, 1 character = 7 bits
  • In Unicode, 1 character = 16 bits
  • UDH = 6 bytes
Message Type Calculation Max Characters per Segment
GSM Single Segment (140 bytes x 8 bits) / 7 bits 160 characters
GSM Multi-Segment ((140 bytes - 6 bytes) x 8 bits) / 7 bits 153 characters
Unicode Single Segment (140 bytes x 8 bits) / 16 bits 70 characters
Unicode Multi-Segment ((140 bytes - 6 bytes) x 8 bits) / 16 bits 67 characters

Bandwidth (and the industry as a whole) counts messages by segments; therefore, customers will be charged for each individual message segment sent to downstream carriers. For example:

Characters used in a message Total number of characters Encoding Message Segments Calculation
Text Only 160 GSM-7 1 No UDH is required, all 160 characters are available
Text Only 240 GSM-7 2 153+87=240 characters
Text and emojis 150 UCS-2 3 67+67+16=150 characters

Why does my message look like GSM characters but is being split as a UCS message?

The most common sneaky characters we see are the "smart quote" (U+2019 to U+0027) as a result of text editors trying to be helpful and "white space" characters (U+2002 to U+0020) that typically surface when copying and pasting. 

Article is closed for comments.