Overview
SMS concatenation is the process that allows a long text message to be split into multiple smaller parts (known as segments) and then reassembled on the recipient’s device as a single message.
Because SMS messages have strict length limits based on their encoding, longer messages must be divided before being sent. Each segment is transmitted separately by the mobile network but displayed to the end user as one continuous message.
Why concatenation is needed
A standard SMS can only contain a limited number of characters:
| Encoding Type | Max Characters (Single SMS) | Max Characters per Segment (Concatenated SMS) |
|---|---|---|
| GSM-7 | 160 | 153 |
| Unicode (UCS-2) | 70 | 67 |
If your message exceeds the limit, it must be split into multiple parts. For example:
A 300-character message using GSM-7 encoding will be divided into two segments (153 + 147 characters).
A 150-character message using Unicode encoding will be divided into three segments (67 + 67 + 16 characters).
Concatenation ensures that these parts arrive in the correct order and are displayed as a single, readable message on the recipient’s phone.
How concatenation works
When a message is too long to fit in one SMS, the system automatically performs the following steps:
Splitting the message
The text is divided into smaller parts, each within the allowed character limit for the chosen encoding.Adding headers
Each segment is given a small header (called a User Data Header, or UDH) that includes information about:The total number of segments
The position of the segment (e.g., part 1 of 3, part 2 of 3, etc.)
This metadata ensures that the receiving device can reassemble the message in the correct order.
Transmission
Each segment is sent individually through the network, using the same destination number.Reassembly on the device
The recipient’s phone reads the headers and automatically combines the segments to display the complete message as one single SMS.
Important considerations
Each segment is billed as one SMS.
Even though the recipient sees a single message, each part counts separately toward billing and message volume.Character count decreases for concatenated messages.
The inclusion of the UDH reduces the maximum number of usable characters in each part (from 160 to 153 in GSM-7, and from 70 to 67 in Unicode).Delivery may vary across carriers or devices.
While concatenation is widely supported, in rare cases some older devices or carriers may not correctly reassemble the message, displaying it as multiple texts instead.Encoding impacts segmentation.
Messages containing special characters (such as accents or emojis) use Unicode encoding, which allows fewer characters per segment. This often increases the total number of segments.
Example
Message:
Thank you for registering! We’re excited to have you with us. To complete your setup, please confirm your email and start exploring your new account today.
Character count: 176
Encoding: GSM-7
Segments: 2 (153 + 23 characters)
The message will be automatically split into two segments and then concatenated by the recipient’s device to appear as one complete message.
Best practices
Keep messages concise to stay within a single segment whenever possible.
Avoid special characters that switch encoding to Unicode.
Use short links instead of long URLs to reduce character count.
If your campaign includes personalization (like names or codes), account for extra characters that may vary between recipients.
Review your message preview in the platform before sending to confirm the number of segments and total character count.
Summary
Concatenation allows long SMS messages to be split into smaller parts and then reassembled seamlessly on the recipient’s phone.
While this feature ensures better readability and user experience, it also increases message segmentation and cost. Understanding how concatenation works helps you optimize your SMS content for efficiency and cost-effectiveness.