SMS Limits

Overview of how Clerk encodes its SMS messages. Encoding is the process of converting data from one form to the other so that it can be translated uniformly.
When you want to send an SMS message you need to understand that there are two different sets of characters that are used by different mobile carriers: GSM-7 and UCS2. When sending SMS messages, Clerk will automatically send messages in the most compact encoding possible. Clerk plans are priced based on your SMS usage (both sent and received).
The character limit for a single SMS message is 160 characters. Nevertheless, most modern phones and networks support concatenation; they can rebuild messages up to 1600 characters. Messages not using GSM-7 encoding are limited to 70 characters.
Example: A 161-character message will be counted as two messages, one with 153 characters and the second with 8 characters.
If you include non-GSM characters, such as an emoji, in SMS messages, those messages have to be sent via UCS-2 encoding. Messages containing any UCS-2 characters are limited to 70 characters.
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.
For example:
  • A message containing text characters only will be encoded using GSM-7
  • A message containing emojis will be encoded using UCS-2
Characters Used
Max characters
Hello - How are you?
GSM Standard
😊 👍 🏕️
Note: We recommend that you check any bulk messages in the Message Segment Calculator before you send them out.

SMS Encoding

Clerk supports both GSM-7 and UCS-2.
GSM-7 is a character encoding standard used for commonly used letters and symbols in many languages.
  • It uses 7 bits to send a single character/symbol on GSM networks. As SMS messages are transmitted as 140 8-bit octets at a time, GSM-7 encoded SMS messages can carry up to 160 characters (140*8/7=160).
  • Refer here for the basic character set for GSM-7.
UCS-2 is a character encoding standard used if a message cannot be encoded using GSM-7 or when a language requires more than 128 characters to be rendered.
  • It uses a fixed length of 16 bits (2 bytes) to send a single character. As SMS messages are transmitted as 140 8-bit octets at a time, UCS-2 encoded messages can carry up to 70 characters {(140*8) / (2*8) = 70}.
  • UCS-2 can encode anything in the Basic Multilingual Plane.

Rate Limits and Message Queues

There are limits to how quickly messages can be sent to wireless carrier networks. This guide explains how Clerk processes your message requests, and the limitations for each phone number type.
When you send out an SMS Campaign, the messages are queued for delivery. The rate at which Clerk dequeues messages varies depending on the origination and destination in the message requests. Below is a quick summary of the default dequeue rate.
Phone Type
Message Segments per Second (MPS)
Maximum Queue Length (Message Segments)
Long code (10DLC)
1 MPS per number
25 MMS/sec
Short code
40 MMS/second
1.5 Media Message /second 25 Text Messages/second
Note: The dequeue rate mentioned above is a rough estimate, and not an SLA for delivery speed. Multiple factors, such as traffic congestion or carrier issues may impact the actual delivery rate.