# GSM 7-bit vs UCS-2 encoding — when each is used

> An SMS body can be encoded in GSM 7-bit (160 characters per segment, covers ASCII plus a small extension table) or UCS-2 (70 characters per segment, covers any Unicode codepoint including Arabic, Chinese, emoji). The choice happens automatically — your aggregator scans the body and picks the narrowest encoding that fits. Mixed encoding is not allowed within a single SMS; one non-GSM-7 character forces the whole message to UCS-2.

- Canonical URL: https://www.broadnet.me/answers/what-is-encoding-gsm-7-ucs-2
- Language: en
- Last reviewed: 2026-05-27
- Publisher: BroadNet (https://www.broadnet.me)

## The 160 vs 70 cliff

If your message fits in GSM 7-bit, you get 160 characters per segment. If you add a single em-dash (—), curly quote (""), or any Arabic / Chinese / emoji character, the whole message becomes UCS-2 and you get 70 characters per segment. A 150-character message that fits in one GSM segment becomes a 150-character message that needs 3 UCS-2 segments — tripling your cost. For high-volume sends, normalise punctuation to GSM-7 ASCII equivalents at composition time.

## Concatenation (multipart SMS)

Messages longer than one segment are split client-side and stitched together by the recipient handset using the User Data Header (UDH). UDH-concatenated segments are 153 chars (GSM-7) or 67 chars (UCS-2) — three bytes per segment are consumed by the UDH itself. Most aggregators bill per segment, not per message — a 320-char GSM-7 message is billed as 3 segments.

## When to deliberately use UCS-2

Always for Arabic, Chinese, Japanese, Korean, Cyrillic, or emoji content. Always for European languages with diacritics not in the GSM-7 extension (Polish ł, Czech ř, Vietnamese ă). Arabic-language SMS in MENA is essentially 100% UCS-2 — budget accordingly (Arabic OTP messages typically run 2 segments).

## The 8-bit option (rarely used)

SMPP also supports raw 8-bit binary encoding for binary data — used for OTA SIM commands, ringtone delivery (1990s), and very niche M2M scenarios. Almost never seen in commercial A2P A2P traffic.

## Related

- [All answers](https://www.broadnet.me/answers)
- [Glossary](https://www.broadnet.me/answers)
- [What is GSM-7 encoding?](https://www.broadnet.me/answers/what-is-gsm-7)
- [UCS-2 — Unicode SMS encoding explained](https://www.broadnet.me/answers/what-is-ucs-2-sms)
- [What is A2P SMS?](https://www.broadnet.me/answers/what-is-a2p-sms)
- [SMS pricing per MT — what you're actually paying for](https://www.broadnet.me/answers/sms-pricing-per-mt-explained)
- [SMS segment calculator](https://www.broadnet.me/tools/sms-segment-calculator)
- [SMS API](https://www.broadnet.me/products/sms-api)
- [Bulk SMS](https://www.broadnet.me/products/bulk-sms)