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

Mara Wallace

Updated May 11, 2023 18:58

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, and the encoding used depends on the content of the message. The HTTP API limits SMS messages to a maximum length of 2048 characters.

See here 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

This chart shows the maximum number of characters that can be sent in a single SMS segment to carriers.

Message	Type	Characters used in message	Encoding	Max characters/ message(without UDH)
Hello - good morning	Text	GSM Standard	GSM-7	160
	Emoji	Unicode	UCS-2	70
	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 GSM-7 or UCS-2 encoding to send the messages and each encoding has limitations to the number of characters that can be sent. The limitations for a single segment are -

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?

Sure can! Here are some important units to start with:

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

Alright, let's do some math!

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 message	Total number of characters	Encoding	Message Segments	Calculation
Text Only	160	GSM-7	1	No UDH 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

How do I know how many segments my message will be?

This handy tool, SMS Split, is a great way to check how your message will be encoded and split into segments.

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. Use the SMS Split tool to determine if you have one of these characters hiding in your message.

Was this article helpful?

Article is closed for comments.