GSM-7 vs Unicode: Why SMS Drops to 70 Characters

Ever typed out a perfect 150-character SMS, added a single emoji, and suddenly your message became two segments? That’s the difference between GSM-7 and Unicode encoding—and understanding it can save you significant money on SMS campaigns.

This guide explains exactly why a single emoji can double your messaging costs and how to optimize your texts for maximum efficiency.

The Short Answer

Encoding	Characters per Single SMS	Characters per Multi-part SMS
GSM-7	160 characters	153 characters per segment
Unicode (UCS-2)	70 characters	67 characters per segment

One emoji = Unicode = 70 character limit instead of 160.

That’s a 56% reduction in available space—from a single character.

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What is GSM-7 Encoding?

GSM-7 (Global System for Mobile Communications 7-bit) is the original character encoding standard for SMS, developed in the 1980s. It uses 7 bits per character, allowing 160 characters in a single 140-byte SMS payload.

The GSM-7 Character Set

GSM-7 supports exactly 128 standard characters plus 10 extended characters:

Standard Characters (7 bits each):

A-Z  a-z  0-9
@ £ $ ¥ è é ù ì ò Ç Ø ø Å å
Δ _ Φ Γ Λ Ω Π Ψ Σ Θ Ξ
Æ æ ß É
! " # ¤ % & ' ( ) * + , - . /
: ; < = > ? ¡ ¿
Space, newline, carriage return

Extended Characters (14 bits = 2 character spaces):

^ { } \ [ ~ ] | € (form feed)

Why 160 Characters?

The math is elegantly simple:

SMS payload = 140 bytes (1,120 bits)
GSM-7 = 7 bits per character
1,120 ÷ 7 = 160 characters

This is why 160 became the magic number for SMS length.

What is Unicode (UCS-2) Encoding?

When your message contains characters outside the GSM-7 set, the SMS system automatically switches to Unicode encoding (specifically UCS-2, a subset of UTF-16).

Unicode uses 16 bits per character—more than double GSM-7’s 7 bits.

The Math Behind 70 Characters

SMS payload = 140 bytes (1,120 bits)
Unicode = 16 bits per character
1,120 ÷ 16 = 70 characters

That’s it. The same physical SMS capacity, but each character takes more than twice the space.

What Triggers Unicode Encoding?

A single non-GSM character forces the entire message into Unicode mode. Common triggers include:

Emojis (Most Common)

😀 😎 🎉 ❤️ 👍 🔥 ✨ 🚀

Every emoji requires Unicode. There’s no workaround.

Non-Latin Alphabets

Chinese: 你好
Arabic: مرحبا
Russian: Привет
Greek: Γειά σου
Hebrew: שלום
Japanese: こんにちは
Korean: 안녕하세요

Special Symbols

™ © ® • … — – ″ ″
Smart quotes: " " ' '
Mathematical: × ÷ ≠ ≈ ∞
Currency: ₹ ₽ ₿ ¥ (¥ is GSM, but ₹ is not)

Accented Characters NOT in GSM

While GSM-7 includes some European characters (é, è, ù, ì, ò, Ç, Ø, ø, Å, å, Ä, Ö, Ñ, Ü, ä, ö, ñ, ü, à), many accented characters require Unicode:

GSM-7: é è ù ì ò à ä ö ñ ü
Unicode required: ê ë î ï ô û ā ē ī ō ū ă ş ț

Multi-Part (Concatenated) SMS

When your message exceeds the single-SMS limit, it’s split into multiple segments. But here’s the catch: each segment loses characters to a header.

How Concatenation Works

┌─────────────────────────────────┐
│ Segment 1                       │
│ [Header: 7 bytes] [Message...]  │
├─────────────────────────────────┤
│ Segment 2                       │
│ [Header: 7 bytes] [Message...]  │
├─────────────────────────────────┤
│ Segment 3                       │
│ [Header: 7 bytes] [Message...]  │
└─────────────────────────────────┘

The User Data Header (UDH) in each segment contains:

Reference number (to reassemble the message)
Total number of segments
Current segment number

This header consumes 7 bytes per segment, reducing available characters:

Encoding	Single SMS	Per Segment (Multi-part)
GSM-7	160 chars	153 chars
Unicode	70 chars	67 chars

Real-World Impact

A 161-character GSM message:

Requires 2 segments
Uses 161 of available 306 characters (153 × 2)
You pay for 2 SMS credits

A 71-character Unicode message:

Requires 2 segments
Uses 71 of available 134 characters (67 × 2)
You pay for 2 SMS credits

The Extended GSM Characters Trap

Ten characters are technically part of GSM-7 but count as two characters because they use an escape sequence:

^ { } \ [ ~ ] | € (and form feed)

Example

"Price: €50"
= 10 characters visually
= 11 characters for SMS counting (€ = 2)

This catches many people off guard, especially with the euro symbol (€).

Cost Implications

Understanding encoding directly impacts your SMS budget.

Scenario: 10,000 Message Campaign

Message A (GSM-7, 155 chars):

FLASH SALE! Get 25% off all items today only.
Use code SAVE25 at checkout.
Shop now: example.com/sale
Reply STOP to opt out.

Encoding: GSM-7
Segments: 1
Total SMS credits: 10,000

Message B (Same content + emoji, 156 chars):

🎉 FLASH SALE! Get 25% off all items today only.
Use code SAVE25 at checkout.
Shop now: example.com/sale
Reply STOP to opt out.

Encoding: Unicode (due to 🎉)
Characters: 156 > 70, so needs 3 segments (156 ÷ 67 = 2.33 → 3)
Total SMS credits: 30,000

One emoji tripled the cost.

How to Optimize Your SMS Messages

1. Avoid Emojis in Business SMS

Yes, emojis add personality. But for transactional or bulk messaging, the cost often outweighs the benefit.

Instead of: Your order shipped! 📦 Use: Your order shipped!

2. Watch for Smart Quotes

Word processors and some phones auto-convert quotes:

Unicode (bad): "Hello" (curly quotes) GSM-7 (good): "Hello" (straight quotes)

3. Use GSM-7 Equivalents

Unicode	GSM-7 Alternative
— (em dash)	- (hyphen)
… (ellipsis)	… (three periods)
× (multiplication)	x (letter x)
” ” (smart quotes)	” (straight quote)

4. Test Before Sending

Always verify encoding before launching campaigns:

Check your message with our free SMS Calculator →

5. Consider Your Audience

If messaging users who primarily speak languages requiring Unicode (Chinese, Arabic, etc.), you can’t avoid Unicode. Factor the higher per-message cost into your budget.

When Unicode is Worth It

Despite the cost, Unicode is sometimes the right choice:

International audiences: Communicating in native languages builds trust
Brand differentiation: A well-placed emoji can increase engagement significantly
Character-limited contexts: Sometimes Unicode symbols convey meaning more efficiently than words

The Engagement vs. Cost Tradeoff

Studies show emojis can increase engagement by 25-30%. If your message generates enough additional conversions, the extra cost may be justified.

Calculate your break-even:

If Unicode costs 3x more (3 segments vs 1):
Engagement must increase by 200% to break even
OR conversion value must be high enough to absorb the cost

Technical Deep Dive: How Detection Works

SMS systems automatically detect encoding using a simple algorithm:

function detectEncoding(message):
    for each character in message:
        if character NOT in GSM_7_SET:
            return UNICODE
    return GSM_7

The check happens before transmission. There’s no “partial Unicode”—it’s all or nothing.

The GSM-7 Detection Set

For developers implementing SMS systems, here’s the exact character set to check against:

const GSM_7_BASIC =
  '@£$¥èéùìòÇ\nØø\rÅåΔ_ΦΓΛΩΠΨΣΘΞÆæßÉ ' +
  '!"#¤%&\'()*+,-./0123456789:;<=>?' +
  '¡ABCDEFGHIJKLMNOPQRSTUVWXYZÄÖÑÜ§' +
  '¿abcdefghijklmnopqrstuvwxyzäöñüà';

const GSM_7_EXTENDED = '^{}\\[~]|€';

function isGSM7(text) {
  for (const char of text) {
    if (!GSM_7_BASIC.includes(char) &&
        !GSM_7_EXTENDED.includes(char)) {
      return false;
    }
  }
  return true;
}

Quick Reference Card

Question	GSM-7	Unicode
Max chars (single SMS)	160	70
Max chars (per segment)	153	67
Bits per character	7	16
Supports emojis?	No	Yes
Supports Latin alphabet?	Yes	Yes
Supports Chinese/Arabic/etc.?	No	Yes
Cost comparison	1x	~2-3x

Don’t Guess—Test Your Message

Paste your SMS text into our calculator to instantly see:

✓ Character count (including extended chars)
✓ Encoding type (GSM-7 or Unicode)
✓ Number of segments
✓ Which characters trigger Unicode