# Encoding detection and SMS cost

> Detecting encoding means checking whether every character is in the GSM-7 alphabet; if even one is not, the whole message becomes UCS-2 and capacity drops from 160 to 70 characters. Because SMS is billed per segment, a single stray emoji or curly quote can double or triple the cost of a message.

- Canonical URL: https://www.broadnet.me/reference/encoding-detection-and-cost
- Language: en
- Last updated: 2026-07-18
- Publisher: BroadNet (https://www.broadnet.me)

How to detect whether text is GSM-7 or UCS-2, which characters silently force UCS-2, and how encoding drives the segment count — and therefore the cost — of every SMS you send.

## How to detect GSM-7 vs UCS-2

The rule is simple and absolute: if every character in the message is present in the GSM-7 basic or extension table, the message can be sent as GSM-7; if any character is not, the entire message must use UCS-2. Detection is therefore a set-membership test against the GSM-7 alphabet, applied to the whole string, remembering that the ten extension characters count as two septets each.

*Detection logic*

```text
for each character c in message:
  if c not in GSM7_BASIC and c not in GSM7_EXTENSION:
     encoding = UCS-2   # one stray char forces the whole message
     break
else:
  encoding = GSM-7
```

## The characters that quietly force UCS-2

The expensive characters are the ones users introduce without noticing. Word processors and phones auto-substitute straight quotes and hyphens with typographic versions that are not in GSM-7. Emoji are never in GSM-7. Non-Latin names and any accented letter outside the GSM-7 accent set do the same. Sanitising these before sending — replacing smart punctuation with ASCII, transliterating where acceptable — keeps messages on GSM-7.

*Common silent UCS-2 triggers and safe swaps*

| Triggering character | GSM-7-safe replacement |
|---|---|
| ' ' (curly single quotes) | ' (straight apostrophe) |
| “ ” (curly double quotes) | " (straight quote) |
| – — (en/em dash) | - (hyphen) |
| … (ellipsis) | ... (three dots) |
| emoji | (remove, or describe in words) |
| non-breaking space | (normal space) |

## How encoding drives segment count

SMS is charged per segment, and encoding sets how many segments a message needs. The thresholds are fixed: GSM-7 gives 160 in one segment then 153 per part; UCS-2 gives 70 then 67. So a 75-character message is one segment in GSM-7 but two in UCS-2. A 160-character promotional SMS with one emoji jumps from one segment to three. Encoding is the largest controllable factor in per-message cost.

*Segment thresholds*

| Segments | GSM-7 chars | UCS-2 chars |
|---|---|---|
| 1 | 1 – 160 | 1 – 70 |
| 2 | 161 – 306 | 71 – 134 |
| 3 | 307 – 459 | 135 – 201 |
| n (n ≥ 2) | 153 × n | 67 × n |

## Practical cost control

Three habits keep SMS cost predictable. First, validate and sanitise every outbound message's characters, flagging anything that forces UCS-2. Second, keep templates within a single GSM-7 segment (≤160 chars) wherever the message allows. Third, measure the realised segment count per message and per campaign, since that — not character count — is what you are billed for. A segment calculator makes the trade-off visible before you send.

## Frequently asked questions

### How do I know if my SMS is GSM-7 or UCS-2?

Check every character against the GSM-7 alphabet. If all characters are in the GSM-7 basic or extension table, it is GSM-7; if any single character is not, the whole message is UCS-2. A GSM-7/Unicode checker automates this and highlights the offending characters.

### How much does one emoji cost me in an SMS?

Potentially a lot. One emoji forces the whole message to UCS-2, dropping capacity from 160 to 70 characters. A 160-character message with a single emoji goes from one segment to three, roughly tripling the cost.

### How is a multi-segment SMS billed?

Each segment is billed as a separate SMS. A message that splits into three parts is charged as three messages. That is why encoding, which sets the per-segment capacity, is the biggest lever on cost.

### How can I keep messages on GSM-7?

Sanitise input before sending: replace curly quotes and dashes with ASCII equivalents, remove emoji, and transliterate stray accents where acceptable. Keep templates within 160 GSM-7 characters so they stay one segment.

### Does character count or segment count determine cost?

Segment count. Two messages with the same character count can cost different amounts if one uses UCS-2. Always measure realised segments per message, not raw character length.

## Sources

- [3GPP TS 23.038 — alphabets and data coding](https://www.3gpp.org/dynareport/23038.htm)
- [3GPP TS 23.040 — SMS technical realization](https://www.3gpp.org/dynareport/23040.htm)

## Related

- [The GSM-7 character set](https://www.broadnet.me/reference/gsm7-character-set)
- [UCS-2 and Unicode SMS](https://www.broadnet.me/reference/ucs2-unicode-sms)
- [SMS segmentation & UDH](https://www.broadnet.me/reference/sms-segmentation-concatenation-udh)
- [SMS segment calculator (free tool)](https://www.broadnet.me/tools/sms-segment-calculator)
- [GSM-7 / Unicode checker (free tool)](https://www.broadnet.me/tools/gsm7-unicode-checker)