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How SMS Message Segments and Character Limits Work
GSM-7 (Standard English)
Single SMS messages hold up to 160 characters. When messages exceed 160 characters, concatenation headers reduce each segment capacity to 153 characters.
UCS-2 (Unicode & Emojis)
Including a single emoji or special character forces 16-bit encoding, immediately lowering segment capacity to 70 characters (or 67 characters for multi-part messages).
Carrier Segment Billing
Mobile networks bill by individual segments, not messages. A 3-segment message sent to 10,000 contacts results in 30,000 billed SMS transactions.
SMS Segment Character Threshold Reference Table
| Segments | GSM-7 Character Limit | Unicode (UCS-2) Character Limit | Header Overhead (UDH) |
|---|---|---|---|
| 1 Segment | 1 – 160 characters | 1 – 70 characters | 0 characters (No UDH) |
| 2 Segments | 161 – 306 characters | 71 – 134 characters | 7 chars / segment |
| 3 Segments | 307 – 459 characters | 135 – 201 characters | 7 chars / segment |
| 4 Segments | 460 – 612 characters | 202 – 268 characters | 7 chars / segment |
Frequently Asked Questions About SMS Segments & Character Limits
Technical answers for marketing managers, developers, and operations teams managing SMS outreach.
How many characters can fit in a single SMS message segment before it splits?
A single standard SMS encoded in GSM-7 can hold up to 160 characters. If the message exceeds 160 characters, it is split into multiple concatenated segments of 153 characters each (with 7 characters reserved for the User Data Header that instructs the recipient's phone how to reassemble the message). If your message contains Unicode characters (like emojis or special accents), each segment is restricted to 70 characters for single messages and 67 characters for multi-part messages.
Why does adding a single emoji or curly apostrophe drastically increase my SMS segment count?
Standard SMS uses 7-bit GSM encoding. The moment a non-GSM character—such as an emoji (😊), a curly apostrophe ('), or a smart quote—is introduced, the entire message must convert to 16-bit UCS-2 Unicode encoding. This reduces the maximum character limit per segment from 160 down to 70 characters, often doubling or tripling the number of billable segments instantly.
What is the difference between GSM-7 and UCS-2 Unicode encoding in SMS messaging?
GSM-7 uses 7 bits per character, supporting standard Latin alphanumeric characters, common punctuation, and a few Greek symbols. UCS-2 Unicode uses 16 bits per character, supporting all global scripts (Arabic, Cyrillic, Chinese, Kanji, emojis). Because Unicode characters require more data bandwidth, fewer characters fit into each 140-byte mobile transmission packet.
How do telecommunication carriers charge for multi-part concatenated text messages?
Carriers and SMS gateway providers (such as Twilio, Sinch, and Telnyx) charge on a per-segment basis, not per message. A text message split across 3 segments will be billed as 3 separate SMS units, even though the recipient sees it as one continuous bubble on their smartphone screen.
What is User Data Header (UDH) and why does a concatenated SMS hold only 153 characters?
When an SMS exceeds 160 characters, the carrier sends it across multiple segments. To ensure the recipient's phone can reconstruct the segments in the correct order, the carrier prepends a 6-byte (48-bit) User Data Header (UDH) to each transmission packet. This reduces the available payload from 160 to 153 characters for GSM-7, and from 70 to 67 characters for UCS-2.
How can marketing and sales teams minimize SMS segment costs without reducing message clarity?
Teams can optimize SMS spend by: (1) Replacing curly quotes with standard straight quotes, (2) Avoiding emojis or placing them only when campaign conversion outweighs segment cost, (3) Using short URLs, and (4) Testing drafts in a free SMS segment calculator before launching mass outreach.
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