Message Segments & Encoding

Every SMS message is split into one or more segments before delivery. The number of segments determines your per-message cost — Trackly bills per segment, not per message.

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What is a Segment?

A segment is a fixed-size chunk of an SMS message. Short messages fit in one segment. Longer messages are split across multiple segments, each with overhead bytes for reassembly headers.

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GSM-7 vs UCS-2 Encoding

SMS supports two character encodings. The encoding is chosen automatically based on the characters in your message. If every character in your message is GSM-7 encodable — in the basic table or the extension table — the message uses GSM-7 encoding. If any character falls outside both tables, the entire message switches to UCS-2.

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GSM-7 Basic Character Set

The GSM-7 basic set includes:

• Letters: A-Z, a-z
• Digits: 0-9
• Symbols: @ £ $ ¥ ! " # ¤ % & ' ( ) * + , - . / : ; < = > ? ¡ ¿ § _
• Accented characters: è é ù ì ò Ç Å å Ä ä Ö ö Ñ ñ Ü ü à Æ æ ß É
• Greek letters: Δ Φ Γ Λ Ω Π Ψ Σ Θ Ξ
• Special: Ø ø and standard whitespace (space, newline, carriage return)

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GSM-7 Extension Characters

These characters are still GSM-7 — they do not force UCS-2 — but each one costs two septets instead of one: ^ { } \ [ ] ~ | € (plus form feed). A message full of them fits roughly half as many characters per segment.

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Characters That Trigger UCS-2

These commonly used characters force UCS-2 encoding, dramatically reducing your per-segment character limit:

• Emoji (any emoji forces UCS-2)
• Smart quotes: \u201c \u201d \u2018 \u2019 (use straight quotes ” ’ instead)
• Em dash: \u2014 (use a hyphen - instead)
• Ellipsis character: \u2026 (use three periods … instead)
• Non-Latin scripts: Chinese, Arabic, Cyrillic, etc.

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Segment Calculation

The platform calculates segments using this algorithm:

1. Check whether every character is in the GSM-7 basic table or the extension table — basic-table characters cost 1 septet each, extension-table characters cost 2 septets each
2. If GSM-7: budget 160 septets (single) or 153 septets per part (multi-part)
3. If UCS-2: use a 70-character limit (single) or 67 characters per part (multi-part)
4. Divide the septet count (GSM-7) or the character count (UCS-2) by the per-segment limit, rounding up. A 2-septet extension character is never split across a part boundary — it moves whole to the next part

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Examples

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Billing Impact

Trackly bills per segment at tiered rates within a 360-day cycle:

• First 1M segments: $0.0045/segment
• Next 4M segments (1M–5M): $0.0030/segment
• Next 15M segments (5M–20M): $0.0020/segment
• Above 20M segments: $0.0011/segment

Segments sent through a BYOC (bring-your-own-carrier) list bill on a separate two-tier rate — $0.0045/segment for the first 1M,$0.0030/segment after — and don’t draw from or count toward the tiers above. A 2-segment message costs twice as much as a 1-segment message. Keeping messages within the single-segment GSM-7 limit (160 characters) is the most cost-effective approach.

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Tips for Reducing Segments

1. Avoid emoji — they force UCS-2 encoding and cut your limit from 160 to 70 characters
2. Use straight quotes — replace smart quotes (\u201c \u201d) with straight quotes (” ”)
3. Use short links — Trackly’s link shortener creates compact URLs that save characters
4. Keep messages under 160 characters — one GSM-7 segment is the sweet spot for cost and deliverability
5. Check segment counts before sending — a sandbox send and the preflight endpoint both return a segment_count field so you can verify segmentation before a live send; the live send response returns only success, message_id, and status

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Template Length Cap

Creative templates are limited to 1,600 characters. This prevents accidentally creating messages that would produce an excessive number of segments.

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Next Steps